We report here that we have built a set of piezomodulated-reflectivity measureme nt system, and obtained piezomodulated-reflectivity spectra of two GaAs/Al0 .29 Ga0.71As single quantum well samples in which the well widths a re 5 and 25nm respectively. From these spectra, the transitions between electron and heavy and light hole subbands can be easily identified. Furthermore, we have ob served the optical transition related to spin-orbit split-off of GaAs buffer. Ef fective mass theory is applied to calculate the transition energy between electr on and hole subbands. Then the spectral structures are assigned based on the cal culated results. It is found that these calculated results agree with experiment al ones very well.