The title compound 3-(4-hydroxy-3-methoxyphenyl)-7,7-dimethyl-7,8-dihydrocinnolin-5(6H)-one (3) was prepared via one-pot three component reaction of 2-(4-hydroxy-3-methoxyphenyl)-2-oxoacetaldehyde with dimedone in the presence of hydrazine hydrate and studied by the single crystal X-ray diffraction method. Its structure was also con?rmed by IR, 1H and 13C NMR spectroscopy. Compound 3 was crystallized in the monoclinic system, space group P21/c, a = 7.921(2) ?, b = 11.566(4) ?, c = 16.986(6) ?, B = 107.338(5)°, V = 1485.5(8) ?3, Z = 4, R1 = 0.0559 and wR2 = 0.1253. The crystal structure of 3 also shows a weak interaction between O3 and N2 atoms.