Recently semiconductor divices have been applied to electric equipments in a radiation environment. The electric proparties of semiconductor devices are known to be sensitive to the lattic deffects produced by irradiation of radiation ray. The effects of thermal neutron radiation damage in various transistors on the market measured about VCE-IC, VCB-IC characteristics and h-parameters. In this paper, the effects of radiation on high frequency transistors and low frequency transistors will be discussed, and results will show that high frequency type transistors are more resistant to radiation damage than low frequency type transistors.