We investigate a novel nonlinear optical nitrophenylhydrazone (NPH) crystal, 3,4,5-trimethoxybenzaldehyde-4-nitrophenylhydrazone (TMO-NPH). Compared with many acentric NPH crystals having rod-shaped heads reported previously, the TMO-NPH molecule maintains the acentric intermolecular hydrogen bond synton through hydrazone and nitro groups but has distinct space filling characteristics due to a different shape of the headgroup, the disk-shaped trimethoxybenzaldehyde. We investigated the crystal structure and its characteristics, as well as details of the microscopic and the macroscopic nonlinear optical properties by quantum chemical calculations using density functional theory (DFT) and powder second-harmonic generation at nonresonant conditions. For comparison, DA-NPH (4-dimethylaminobenzaldehyde-4-nitrophenylhydrazone) having a rod-shaped head and known nonlinear optical properties is also investigated by the same analysis. TMO-NPH crystals have the monoclinic space group symmetry Pn and exhibit a large macroscopic optical nonlinearity due to a favorable molecular packing with a very high order parameter cos(3) (theta(P)) = 0.90, while DA-NPH exhibits a lower order parameter cos(3)(theta(P)) = 0.33. The diagonal effective hyperpolarizability tensor component of TMO-NPH crystals is 30% higher than the largest tensor component of DA-NPH, even though the microscopic molecular susceptibility of TMO-NPH molecule is only about half of the one of DANPH.