Cu2ZnSnSe4 is a promising material for absorber layers of thin film solar cells, since Cu2ZnSn(S,Se)4 thin films have the optical band-gap energy of 0.94 - 1.5 Ev, large optical absorption coefficient of 104 cm-1 and all constituents of these films are abundant and non-toxic [1]. Recently, CZTSSe-based thin film solar cells of 10.1 % efficiency have been made by hydrazinebased solution processing for the first time [2]. However, in literature there is no data on the crystal structure of Cu2ZnSnSe4 obtained by single crystal x-ray diffraction measurements. The results of powder X-ray diffraction (R indices equal to 0.0517) [3] showed that Cu2ZnSnSe4 crystallizes in a stannite structure. At the same time, kesterite type structure for Cu2ZnSnSe4 was established by neutron diffraction investigations [4]. The Cu2ZnSnSe4 crystals are formed via peritectic reactions at 1063K [5], which complicates their preparation by crystallization of the melt. We have prepared crystals of the Cu2ZnSnSe4 compound using the Bridgman method. To avoid peritectic reaction and appearance of ZnSe as a second phase during crystallization process, the synthesis of Cu2ZnSnSe4 from initial elements and the subsequent crystallization were carried out at the temperature 50K - below the temperature of peritectic reaction. The obtained single crystals were used for the X-ray analysis. The X-ray diffraction data were collected from a single crystal of the size 0.1×0.06×0.03мм3 at room temperature using an Xcalibur E diffractometer supplied with an EOS CCD space detector and a monochromatic source of MoKα radiation (graphite monochromator). The data were collected and processed using the program CrysAlisPro (Oxford Diffraction Ltd., version 1.171.33.66) and were corrected for the Lorentz and polarization effects, and absorption. Final parameters for tetragonal unit cell of Cu2ZnSnSe4 were obtained using an entire data set and equal: a=b= 5.6884(4)Å, c= 11.3472(13)Å, V=367.17(7) Å3 The structure was refined both in space groups I42m (stannite model ) and I-4 (kesterite model) by the full matrix least squares method based on F2 with anisotropic displacement parameters using the program SHELXL 97 [6]. The refinement shows that the stannite model (space group I-42m) better fits the diffraction data. The corresponding R indices are 0.0234 and 0.0292 for the stannite and kesterite model, respectively. The refinement of the stannite model showed that cooper and zinc atoms alternate in d Wickoff position of the unit cell and statistically occupy it with equal probability. The contradiction of our results to neutron diffraction measurements and their agreement with early reported powder diffraction measurements can be explained by the influence of the crystal preparation technology on the type of crystal structure (stannite or kesterite) of Cu2ZnSnSe4 samples. Financial supports from the STCU # 5402 and IRSES PVICOKEST – 269167 projects are acknowledged.