We have measured the precise lattice parameter of undoped and impurity-doped ZnSe single crystals by the Fewster method. The lattice parameter of undoped ZnSe single crystals was 0.566919 +/- 0.000002 nm and it remained unchanged after the Zn-treatment at 1000 degrees C. On the other hand, dilation of lattice parameter observed in I-doped ZnSe depended on Zn-treatment temperature, Increase of lattice parameter also occurred in Al-doped ZnSe. These results suggest that I and Al impurities in ZnSe crystal play an important role with point defects formation during and after the Zn heat treatment.