Micro-photoluminescence (μ-PL) measurement w as performed on GaAs/AlGaAs quantum well infrared photodetectors (QWIPs). The PL peaks in the multi-quantum wells (MQWs), which reveal the energy positions of band-to-band transitions in the barrier and the well, respectively, are directly related to the Al component in the barrier of Al_xGa_1-xAs. Us ing standard effective mass theory for quantum wells the realistic Al compone ntx and well widthd were deduced. Consequently the peak response wavel ength λ_p of the QWIP was determined. The calculated λp is coincide nt with the result from photocurrent spectrum for the same sample very well. It proves that the present method is very applicable for the MQW materials con trol in the process of device fabrication.