A new method is presented to measure the thermal resistance (Rth) of semiconductor lasers. We show that knowing the applied junction voltage, the electrical series resistance and the experimental Ith vs. T characteristic in pulsed operation is possible to evaluate Rth. The key advantage of the method is that it avoids the measurement in continuous-wave (CW) operation. In particular we consider the case when Ith versus T characteristics exhibit kinks. To verify our method, CdZnSe, InGaAs, GaN and AlGaAs lasers were analyzed. We show that values obtained in this way agree quite well with the values measured by traditional methods.