Using deep level transient spectroscopy (DLTS) theXconduction‐subband energy levels in an AlAs well sandwiched by double GaAs layers were determined. Calculation gives eight subbands in the well with well width of 50 A˚. Among them, five levels and the other three remainders are determined by using the large longitudinal electron effective massm1(1.1m0) and transverse electron effective massmt(0.19m0) atXvalley, respectively. Two subbands with the height energies were hardly detectable and the other six ones with lower energies are active in the present DLTS study. Because these six subbands are close to each other, we divided them into three groups. Experimentally, we observed three signals induced from the three groups. A good agreement between the calculation and experiment was obtained. ©1995 American Institute of Physics.