The title compound 1,2-dihydro-3-methylpyrazole-5-one was determined by X-ray crystal structural analysis. The crystals are monoclinic, with space group P2(1)/n with a = 7.968(7), b = 6.502(6), c = 9.986(10) angstrom, alpha = 90 degrees, beta = 109.995(15)degrees, gamma = 90 degrees, V = 486.1(8) angstrom(3), Z = 4, F(000) = 208, D-c = 1.340g cm(-3), mu = 0.100mm(-1), and the final R = 0.0373 and wR = 0.0961. A total of 4776 reflections were collected, of which 1148 were independent (R-int = 0.0589). In the crystal packing diagram, intermolecular N-H center dot center dot center dot O hydrogen bonds and pi-pi stacking interactions stabilize the solid state of the title compound.