The overall phase velocity change of SAW for a n-Si thin layer on a YZ-LiNbO3 substrate is calculated considering mechanical and electrical perturbations in the case of low-kd limit. The mechanical perturbation is modified to include the dependence on electron concentration in Si. The variation of overall SAW velocity with electron concentration, layer thickness, and frequency show markedly different dependence from that of both mechanical and electrical perturbations. The rapid decreases of overall SAW velocity at low and high electron concentrations are attributed to the electrical perturbation and carrier concentration effect on mechanical perturbation respectively.