The linear and nonlinear optical properties of a high-efficiency organic nonlinear optical crystal, 3-methoxy-4-hydroxy-benzaldehyde (MHBA), are investigated. The principal refractive indices at λ = 532 nm are nx = 1.70331, ny = 1.5624, and nz = 1.81376. Calculations show interesting critical phase-matching properties (frequency doubling of the Nd:YAG laser at 1064 nm and diode laser at 830 nm). The nonlinear optical coefficients, d21 = 25dKDP36 ± 10%, d22 = 10dKDP36 ± 10%, d23 = 33.3dKDP36 ± 10%, d24 = 8.3dKDP36 ± 20%, were determined. MHBA has potential as a practical frequency doubler of Nd:YAG and diode lasers.