A crack-free ZnGeP2 single crystal of diameter Phi 8 and 40 mm length, with FWHM of 39.6 '', was grown using the ACRT Bridgman method. The absorption of ZnGeP2 in the near and middle infrared spectrum was studied at different thermal annealing conditions, i.e., in vacuum, P atmosphere and ZnGeP2 atmosphere conditions, respectively. Annealing in vacuum at 600 degrees C for 300 h, the transmittance near 1 mu m is increased while the transmittance beyond 1.6 mu m is decreased. Annealing in 3 atm P atmosphere at 600 degrees C for 300 h after annealed in vacuum, the transmittance is increased in the whole range. Annealing in ZnGeP2 atmosphere at 600 degrees C for 300 h after former two thermal treatment steps, the transmittance is further increased. The effects of thermal annealing are discussed in connection with possible mechanisms of point defects related optical absorption in ZnGeP2. (C) 2010 Elsevier B.V. All rights reserved.