The memory nature and mechanism of the Ta 2 O 5 -gate-dielectric-based organic phototransistor memory (OPTM) have been studied. The UV–Vis absorption spectra and the X-ray photoelectron spectroscopy indicate that Ta 2 O 5 owns positive interfacial charge because of the existence of Ta–OH. The hydroxide results in oxygen deficiency in Ta 2 O 5 which is proposed to trap electrons. The characteristics of Ta 2 O 5 -based capacitor and the energy level alignment at Ta 2 O 5 –pentacene interface reveal that the electron-injection process is favorable which stimulates the electron-trapping process in Ta 2 O 5 . The Kelvin probe force microscopy of the Ta 2 O 5 -pentacene interface certificates the electron-injection and electron-trapping processes as well. It is the positive charges in Ta 2 O 5 and energy level alignment that lead to the memory effect of Ta 2 O 5 -gate-dielectric-based OPTM. Compared to Ta 2 O 5 , polymethyl methacrylate (PMMA) does not have so strong a positive interface. Accordingly, PMMA films of different thickness are adopted on Ta 2 O 5 to tune the Ta 2 O 5 -pentacene interface, offering control of the memory properties including the memory window and retention time. The understanding of the mechanism is at the forefront of devising high-performance OPTM devices.