A low-coherence laser interferometry has been proposed to measure temperature of power devices. The transient thermal characteristic in power transistors has been measured in real time with a InGaAsP LD used as a microprobe. The experimental results show that this method is accurate and noninvasive. It also has the advantages of high spatial resolution, quick response speed, and a wide measuring range. It is applicable to power electronics and optoelectronics devices fabricated with Si, GaAs, and InP materials.