A method to determin the grating period of periodically poled crystal accurately was reported by using angle and temperature tuning characteristics of quasi-phase matched optical parametric oscillators(OPOs).Based on periodically poled LiNbO_3(PPLN)-OPO pumped by a 1 064.3 nm CW-diode-end-pumped Nd:YVO_4 laser,the grating periods of three PPLN samples were obtained.With the comparison between the theoretical temperature tuning curves and the experimental one,this method is proved to be an effective way to determin the exact grating period of periodically poled crystal.