Al‐Ta‐O composite dielectric thin films prepared by rf plasma sputtering under various conditions have been used for gate insulators of CdSe thin‐film transistors. The hot electron injection method has been used for investigation of the electron trap properties in these thin‐film transistors. It shows clearly that the traps are mainly due to Coulomb attractive centers, i.e., oxygen vacancies. In the best films prepared in this work, capture cross sections of 3.8×10−13cm2and trap densities of 1.0 ×1017cm−3are obtained. The relationships between gate insulator preparation conditions and the traps of the Al‐Ta‐O/CdSe thin‐film transistors are precisely described in this paper.