In this paper a measurement method about optical path change vs.the temperature in a CO 2 laser coupler is described.Using a probe beam through the coupler with two parallel surfaces,we measured the variety of laser power after the coupler while the temperature is changed.Thus a formula for analysing state of the coupler is derived.Then we discussed coupler performences through 1.5 kW and 2.0 kW laser power seperately.The results are in agreement quite good with experiment data.