By scanning the cavity length of optical parametric oscillator (OPO) with tri angle wave voltage of different scan rate applied on PZT, the thermal effect of nonlinear crystal in OPO was observed. The experimental results show that the thermal effect seriously affects the peak power of output pulses at the resonance points, therefore it can be used to demonstrate the thermal self frequency locking of the OPO. The physical mechanism of the observed phenomena is reasonably analysed.