A new technique is described in which a transient thermoelectric (TTE) voltage of a semiconductor is measured after a pulsed laser irradiation—a modification of photodiffusion or Dember effect. This technique has been successfully applied to test samples ofn‐ andp‐Si over the wide time range from 20 ns to 2 s and the temperature 120–284 K. The decay curves of the TTE voltage consist of three stages with the characteristic relaxation or decay times. Stage 1 with the relaxation time &tgr;1, is associated with the carrier generation and recombination and explained by the Dember effect. Stage 2 characterized by the relaxation time &tgr;2, is due to the diffusion of majority carriers from a higher to a lower temperature region; forp‐Si, a double‐relaxation process is observed arising from the difference in the drift mobilities of light and heavy holes. At stage 3, the TTE voltage vanishes completely, which can be reasonably interpreted by the diffusion of thermal flux or phonons along a temperature gradient produced by the laser pulse. Analytical expressions for each stage are presented to discuss these experimental results.