Based on the formula of the time dependence of laser-induced voltage in high Tc superconductor(HTSC) and collosal magneto-resistance (CMR) thin films, we have successfully deduced the function of the optimum thickness for producing largest laser-induced thermoelectric voltage. It is indicated that a direct proportional relationship between laser-induced thermoelectric voltage and the reciprocal of thickness does not correctly describe the situation. In fact, an optimum thickness must exist. This function provides the main factor of affecting the design for fast-time-response device. We have discussed the parameter of thin film influencing the optimum thickness, and compared it with experimental result.