Unusual electrical instability caused by light illumination in double heterojunction pseudomorphic high-electron mobility transistor (DH-wpHEMT) epitaxial wafers is reported for the first time. The principal parameters of DH-pHEMT namely, sheet carrier concentration, sheet resistance, and electron mobility, change slowly with time after illumination is shut off at 300 K. The relaxation time of this transit is estimated to be longer than 10 min for some samples, grown by solid source molecular beam epitaxy. This phenomenon is called quasi-persistent photoconductivity (QPPC). The relationship between QPPC and interface roughness is investigated. Through atomic force microscopy measurements on both upper AlGaAs/InGaAs interface and lower InGaAs/AlGaAs interface of the DH-pHEMT, the lower interface of InGaAs channel is found to be much rougher than the upper one. It is concluded that control of interface smoothness of the lower heterointerface is essential in order to obtain high-performance DH-pHEMT epitaxial wafers with high reproducibility.