The effect of lattice mismatch between the substrate and the heteroepitaxial film on the initial process of ultra-thin film growth, such as the nuclear density, the average size of cluster, the scale behavior and the growth mode, is studied by the kinetic Monte-Carlo method. It is found that the negative (positive) mismatch, corresponding to the compressive (tensile) strain, can induce the growth process transferring from the nucleation stage to the intermediate one earlier (later). The larger the mismatch, the more apparent the above behavior. Under the same deposition condition, the negative mismatch can bring about lower nuclear density and larger average cluster size than the positive mismatch does. Furthermore, the nuclear density is found to follow the power law of Ns≈(F/D)χ, andχdecreases from 0.37 to 0.33 while the mismatch changes from –0.04 to 0.02. This behavior indicates that the growth mode of the ultra-thin film changes from including dimer diffusion into excluding dimer diffusion. Additionally, the scale theory of the cluster size is found to be valid even for the heteroepitaxial growth.