We studied the Multiphoton Ionization Dissociation (MPID) time-of-flight mass spectra of oxirane with ultrasonic molecular beam system at 308 nm and 355 nm laser radiation. In our experiments, the mass spectra distributions showed a certain extend of difference between two conditions. Under 355 nm laser radiation, there is no molecular ion signal and under 308nm we got a high laser intensity index for each fragment ions. Differing from electron impact ionization, the most intensive ion signal came from CHO+. With the laser intensity dependence of each fragment ion and its branch ratio we investigated the PMID process and mechanism of Oxirane at these two laser radiations.