An electro-optic sampling system utilizing proton-bombarded GaP crystal as probe material has been built. Microwave signals propagating on the indium -tin oxide coplanar waveguide were measured by the system. Measurements of bombarded sample gave the resistance value of four orders greater than that of un- bombarded. The electric field shield effect induced by doped GaP was effectively decreased to approximate semi-insulator material. The system has the voltage sensitivity of about 40 mV/2~(1/Hz) with the microwave frequency of 1.15 GHz. An electro-optic sampling system utilizing proton-bombarded GaP crystal as probe material has been built. Microwave signals propagating on the indium -tin oxide coplanar waveguide were gave by the system. Measurements of bombarded sample gave the resistance value of four orders greater than that that The system has the voltage sensitivity of about 40 mV / 2 ~ (1 / Hz) with the microwave frequency of 1.15 GHz.