Crystal structure in an highly oriented organic thin film was determined using two dimensional grazing incidence X-ray diffraction (2D-GIXD). α,ω-Hexyl-distyryl-bithiophene (DH-DS2T) was chosen as the material for the structural analysis, because it is a typical organic semiconductor showing a high crystallinity and an ordered layer-growth behavior. A 2D-GIXD pattern over large range of scattering angles was obtained by using high-brightness synchrotron radiation in SPring-8 and high-sensitive 2D X-ray detector (PILATUS 300 K). The analysis of the observed 2D-GIXD pattern was made to clarify that the crystal structure of highly oriented DH-DS2T thin films belongs to a monoclinic unit-cell with a = 0.58 nm, b = 0.78 nm, c = 3.43 nm, and β = 94.3°, and the space group is determined to be P2₁/a by considering the extinction rule. Furthermore, by fitting the simulated Bragg peak intensities to the experimental data, the molecular structure is determined. In this structure, two molecules are included in the unit cell and they tilt about 25° against substrate normal.