Abstract: Optical fiber used as a sensor converting radiation energy into Oerenkov light energy and a transmission line can form wide band electron and gamma radiation diagnostics system. Application of optical fiber will considerably improve frequency response characteristics of measurement system. Present system is able to break the record of bandwidth limit of routine systems. In this paper,the sensitivity of optical fiber to Oerenkov light emitted in itself by incident relativistic electron beam and the sensitivity of the optical fiber to Oerenkov light emitted by Compton electrons created by incident gamma ray are theoretically calculated. The mathematical expressions for sensitivity, dynamic range and bandwidth of the system are given. These forulas are of great importance in estimating system performances and demonstrating its applicability. The system has maximum sensitivity at the wavelength of 800nm. We have also performed an experimental calibration for sensitivity of the sensor to γ-ray, and a recommendation value for the sensitivity is given. Its bandwidth is expected to be more than 500 MHz. When the system is used to measure very fast pulsed radiation, it shows its distinctive advantage.