ZnGeP2 crystal is one of the most promising materials for infrared nonlinear optical applications. However, the near-band-edge absorption of native defects degrades the performance of ZGP-OPO device. The single-zone-annealing furnaces were utilized to anneal ZGP samples. The optimal annealing temperature was 600 degrees C and the optimal annealing atmosphere was ZGP powder vapor. For further annealing effects, the two-zone-annealing method was adopted to anneal ZGP crystals for the first time. The absorption coefficient can be reduced to 0.06 cm(-1) at 2 mu m and the resistivity can be increased from 5.8 X 10(7) Omega cm to 4.1 X 10(9) Omega cm. The results indicate that the annealed ZnGeP2 crystals could be used as OPO devices. The mechanism of the two-zone-annealing effects has been briefly discussed in this paper. (c) 2012 Elsevier B.V. All rights reserved.