We introduced a new technique called “Transient Thermoelectric Effect (TTE)”, which was based on carrier-diffusion as irradiated by a pulsed laser. The observed TTE voltages decayed exponentially with time, showing a multi-relaxation process with characteristic relaxation times τi(i=1,2,3,…) for thermal diffusions of photo-induced conduction carriers, analyses of these results could give valuable information about carrier mobilities, effective masses and Fermi surfaces. The principle and the main analysis method were also described in detail.