This article investigates the failure due to collapse of the MEMS beams in capacitive MEMS power sensor based on GaAs MMIC process. Generally, the residual stress and adhesion are the main causes of the collapse of the MEMS beams. So, this article investigates the residual stress of the MEMS beams with an improved rotating technique. In addition, a resonant method has been used to investigate the adhesion of the MEMS beams. The measurement result shows that the buckling due to the residual stress is very small. Also, the finite analysis results show the rotating angle theta should be less than 0.12 degrees. Thus, the collapse of the MEMS beams is mainly caused by adhesion.