A high quality ZnGeP2 (ZGP) single crystal 22 mm in diameter and 70 mm in length was grown by a modified vertical Bridgman method. The as-grown and annealed crystals were characterized using XRD, IR spectrophotometers, positron annihilation lifetime (PAL), Hall Effect and resistivity measurements. It is found that there was a cleavage face (2 0 4) along the appearance face on the as-grown crystal. After annealing the crystal, the width of XRD rocking curve of the annealed crystal was decreased, the intensity was high and the symmetry was better than that of the as-grown crystal. The IR transmission spectrum indicated that annealing decreases significantly the optical absorption in the region of 1.3-2.6 mu m and transmittance exceeds 57% in the range of 3-8 mu m. PAL results showed that single zinc vacancies were unstable since they aggregated with each other after annealing under zinc vapor at 500 degrees C, and both the number of single zinc vacancy and the size of large vacancy decreased under cover-up with ZGP polycrystalline powder at 600 degrees C. The Hall effect and resistivity measurement results showed that the ZGP crystal has p-type conductivity; after annealing with ZGP polycrystalline powder the carrier concentration was decreased to 10(10) cm(-3) and resistivity was increased to 10(8) Omega cm. The above mentioned results demonstrated that annealing treatment improved significantly the optical quality of as-grown ZGP crystal; the proposed annealing process is effective and a promising new method to decrease of the point defects in the ZGP crystal. (C) 2010 Elsevier B.V. All rights reserved.