Abstract: The epoxy resin (EP) encapsulant insulation for medium-frequency transformer in power electronic transformers (PET) is subjected to the long-term square wave voltage stress and thermal stress caused by temperature rise, which is a key restriction to the reliability of PETs. To analyze the effect of temperature on the electrical strength of EP under square wave voltage, the partial discharge (PD) and breakdown (BD) characteristics of EP specimens under a 1 kHz 50% duty cycle unipolar square wave from room temperature to 110 ℃ are studied. The Weibull distribution is applied to fit to the measured PD inception field strength (PDIE) and breakdown field strength (BDE). Experimental results show that, below 80 ℃, BDE decreases slightly and PDIE increases slightly with increasing temperature; however, at 110 ℃, BDE significantly decreases and PDIE decreases to 5.44 kV/mm. Combined with free volume breakdown theory of polymers, the variance of BDE at glass transition temperature is explained. Statistical analysis of PD events shows that PDs are symmetrically distributed on the rising and falling edges of the square wave. Small signal PDs are frequently and dispersively located in phase domain, while large signal PDs are relatively concentrated in phase domain. The maximum PD magnitude decrease first and then increase with increasing temperature. PD amplitude-phase distribution patterns similar to experimental results are obtained through a PD stochastic simulation model. It is found that the temperature affects PD delay by changing the detrappable electron emition rate, further determines the PD magnitude and PD dispersity in phase domain.