The influences of gamma irradiation on CdS/CdTe photodiodes were investigated in order to evaluate their potential for applications to the radiation tolerant field emitter array (FEA) image sensor. It was found that the CdS/CdTe photodiodes have sufficient tolerance to high gamma-ray exposure more than 2 MGy. Furthermore, $I {-} V$ characteristics of CdS/CdTe photodiodes under gamma-ray irradiation were investigated. In the CdTe thickness of 6.5 $m\textbf{m}$, the change rate of current density increased with increase in the reverse bias voltage. On the other hand, in the case of 2.2 $m\textbf{m}$, the change rate of current density was almost constant irrespective of the reverse bias voltage. These results suggest that the increase in depletion layer width affects the change rate of current density.