We report on the 180° domain switching characteristics and fabrication of periodically poled KNbO3 for second-harmonic generation. Using a modified triangular pulse, a periodically poled KNbO3 of very high quality was fabricated with dimensions of 3×4×0.925 (a×b×c) mm3 and a period of 32.5 μm to utilize the d31 component of the second-order nonlinear optical coefficient tensor. An efficient second-harmonic generation of a mode-locked Nd:YAG laser, operated at 1064 nm with a pulse width of 35 ps, was achieved with a maximum conversion efficiency of 34.8%. The effective nonlinear coefficient, measured in the limit of undepleted pump approximation, was 5.2 pm/V, which is 68% of the theoretical value. © 2002 American Institute of Physics.