Twinned TATB nanobelts were directly synthesized using a water/AOT/toluene microemulsion system. Here, urea has been used as an aminating agent, which has been proven to play an important role in the formation of such a nanostructure. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and high performance liquid chromatography (HPLC) were used to characterize the composition of the as-synthesized samples. Electron microscopic studies indicated that the morphology and size of the TATB nanobelts can be readily tuned by varying experimental parameters. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) studies showed that the TATB nanobelts possess mirror symmetry, each of which grow along the [001] direction. A possible mechanism was also proposed to account for the growth of these twinned TATB nanobelts prepared by the microemulsion system.