Thermal effects in a 2-μm thick GaAs cantilever beam of the power sensor microsystem are investigated. The increased thermal sensitivity of the microsystem to the thermal conductance changes of the ambient atmosphere is evaluated by the experiment and simulation. A bimetallic effect in the microsystem cantilever beam is studied using both the microscopic laser optical interferometry and laser optical reflectance measurement. The cantilever beam deflections induced by the differential thermal expansion of the cantilever layers are found to be linear with the power dissipated in the microsystem MESFET-heater. The key microsystem transfer characteristics based on the bimetallic effect are obtained.