Abstract Strains in heteroepitaxial semiconductor film of ZnSe and ZnS x Se 1- x grown on (001)-GaAs substrates have been investigated by using the 2 θ/ θ mode of the X-ray diffraction method. Firstly, a method of strain measurement is explored. In the ZnSe films, strains parallel to the interface are expansive for growth temperatures ( T G) above 300°C, while compressive below this temperature. It is found that thermal and misfit strains contribute to the residual strains, and the former is dominant for T G>300°C, whereas the latter for T G>300°C. In the ZnS x Se 1- x films grown at 450°C, the parallel strains are expansive and the magnitude varies with increasing fraction x. It is also found that coherent growth occurs at x=0.05 which corresponds to the misfit of 1.3×10 -3 at T G=450°C.