A new form of fullerene-type carbon, named carbon nanoflasks, is described in this paper, using Co(CO) 3 NO, a special precursor, in their synthesis. Upon its decomposition, the Co(CO) 3 NO is not only a source of carbon, but also gives rise to fcc cobalt particles, whose size can grow from several nanometers to hundreds of nm in the course of the decomposition. After a careful purification process, the percentage of cobalt-filled carbon flasks can be as high as 30%. The width of the flask tube-necks ranges from 50 nm to over 100 nm, while the body of the flask ranges from 100 nm to 500 nm, and in some cases reaches over 1 µm. HRTEM reveals that the graphitic layers of the flask walls are usually over 100 nm thick, and are much thicker than the flask cap. After an acid treatment of the sample, opened and empty carbon flasks can be easily obtained. XRD, TEM, SEM, XPS, AFM, and SQUID measurements were employed in the characterization of the flasks. An explanation of the formation of the carbon nanoflasks is proposed in this paper.