A quadrupled output of a Q-switched Nd:YAG laser (266 nm) was used for ablating semiconductor materials. The ions formed during ablating were analyzed by using a reflective time of flight mass spectrometer. Some characteristics of laser ablation of semiconductor materials, such as ion yields versus laser power, the threshold of laser irradiance, the laser power dependence of the resolution of mass spectra and the shift of peak positions were reported. Experimental results showed that P2+ dimmer dominates in all mass spectra of InP. Typical mass spectra of Al0.3Ga0.7As and InP are shown.