The thermal resistances of high power laser bars are measured utilizing the change of the LD’s PN junction with the temperature.The thermal resistance of each layer on the heat transfer path can be obtained by the structure function theory after collecting the curve of the temperature rise.Optical conversion efficiency can be obtained by measuring thermal resistances of laser bars,and thermal resistances decrease and optical conversion efficiency increases with the increase of the current.The measured results are consistent with the results measured by electrical method.