Abstract Admittance Spectroscopy (AS) measurements have been performed on n+-on-p Hg1-xCdxTe(x=0.6) photodiodes. Before gamma-irradiation, a deep level located 0.15 eV above the valence band is observed, its trap density Nt=4.8×1015cm-3 with the capture cross section of approximately 2.2×10-17cm2, which is probablely induced by a compound defect correlated with Hg vacancy. After 104 Gy gamma-irradiation, a new trap center located 0.19 eV above the valence band is found, and the pre-irradiation trap level 0.15 eV above the valence band is no longer seen. The trap densities for these two levels are almost the same. This defect level change ultimately makes the device performance deteriorated.