The dynamic current-voltage (I-V) characteristics of mid-wavelength HgCdTe photovoltaic detectors under steady-state gamma irradiation have been measured as a function of gamma dosage. Two obvious effects were observed due to gamma radiation. One is the ionization effect demonstrated by the generation of the photocurrent in the diodes. The other is the displacement damage effect reflected by the increased resistivity in the neutral region. Both effects showed dosage dependence. Qualitative analysis showed that the photo electron yield became small with increasing gamma dosage, which meant the radiation-induced defects played a significant role in large dosage range.