Diamond nanocrystals containing highly photoluminescent color centers are attractive, nonclassical, and near-field light sources. For near-field applications, the size of the nanocrystal is crucial, since it defines the optical resolution. Nitrogen-vacancy (NV) color centers are efficiently created by proton irradiation and an-nealing of a nanodiamond powder. Using near-field microscopy and photon statistics measurements, we show that nanodiamonds with sizes down to 25 nm can hold a single NV color center with bright and stable photoluminescence. Probing an electromagnetic field at the nanoscale level is a challenging field of research [1]. Recently, single fluorescent molecules at room temperature have been used to probe the near field of a resonant antenna positioned at the end of a metal-coated glass-fiber near-field probe [2]. Fluorescent-active tips consisting of a single fluorescent nano-object at-tached to a tip offer many prospects in near-field scanning optical microscopy (NSOM), since the spa-tial resolution is expected to scale down to a dimen-sion approaching the size of the emitter, i.e., around 10 nm for a single CdSe nanocrystal [3]. Moreover, such an active tip is a unique technique to study the interaction of a nano-object with its environment [4]. This should open further perspectives in nano-optics and quantum-optics experiments. Several kinds of fluorescent-active objects have been considered, such as a single terrylene molecule in a p-terphenyl microcrystal [5], rare-earth-doped glass particles [6], CdSe nanocrystals [3,7,8], color centers in a thin LiF layer [9], and nitrogen-vacancy (NV) color centers in a diamond nanocrystal [10]. So far, none of these has been satisfactory for a practical use in NSOM. Although near-field imaging using single molecules at cryogenic temperatures is very successful [5], it can hardly be extended at room tem-perature, owing to photobleaching [11]. Experiments using CdSe nanocrystals offered insufficient control over the very object attached to the tip [3], and opti-cal imaging with them turned out to be restricted by blinking [8]. In addition, the achieved spatial reso-lution is similar to conventional NSOM, owing to ei-ther the large size of the material hosting the emitter and/or the number of active centers that is required for optical detection. Ideally, a fluorescent nano-object should combine several properties for application as a practical active tip: It should emit in ambient conditions, have a high quantum efficiency, exhibit a radiative lifetime short enough to allow for a large counting rate at the single emitter level, neither blink nor bleach over a long measurement duration, and hosted in a material of nanometric dimension as small as possible. The nano-object should eventually host a single emitter with known lifetime and dipole orientation. NV − color centers in diamond [12] combine many of these demanding characteristics. Their photolumin-escence centered at 670 nm at room temperature has a near-unity quantum efficiency and corresponds to a radiative lifetime of 11 ns. These color centers are ex-tremely stable emitters; no blinking or bleaching has been reported so far, to our knowledge, for a single emitter. Photon antibunching [13] and single-photon emission [14] have been demonstrated for a NV color center in a single nanodiamond. Such photoluminescent nanoparticles have already been successfully used as a nanoscopic light source for NSOM [10], but the achieved resolution was about 300 nm, a value presumably limited by the size of the crystal attached to the tip. Significant improvement in resolution requires much smaller photolumines-cent nanodiamonds compared with this pioneering experiment. In this Letter, we demonstrate that a single NV color center can be detected in individual nanodia-monds with a size smaller than 25 nm in diameter, i.e., the smallest size reported so far for emitting nanodiamonds, with the same stable, nonbleaching luminescence as in larger particles. This is achieved by combining a NSOM, which offers a dual topo-graphical and optical operating mode, with a photon-correlation measurement. This combination gives ac-cess in a single setup to the size of the particle, its March 15, 2008 / Vol. 33, No. 6 / OPTICS LETTERS 611 0146-9592/08/060611-3/$15.00 © 2008 Optical Society of America