The title compound 4-oxo-1,2,4-triazino[4,5-a]benzimidazole (C9H6N4O, Mr= 186.18) has been synthesized and structurally characterized by IR, NMR and single-crystal X-ray diffraction. The crystal belongs to monoclinic, space group P21/c, with a = 9.1530(18), b = 7.2260(14), c = 12.604(3) Å, β= 106.92(3), V = 797.5(3)Å3, Z = 4, Dc = 1.551 g/cm3,λ(MoKa) = 0.71073, F(000) = 384,μ(MoKa) = 0.109 mm-1, the final R = 0.0632 and wR = 0.1095. A total of 1444 unique reflections were collected, of which 767 with I > 2σ(I) were observed. The structure of the title compound is planar. In packing, the molecules are intersected to each other to form chains along three dimensions, together by intermolecular N–H...O and C–H...N hydrogen bonds. The whole structure is further stabilized by π...π between two adjacent tricyclic ring systems, with the centroid-to-centroid distance of 3.369(4) Å.