The charge-carrier mobility of an organic semiconducting material determines the material potential applications in devices. The investigation on mobility of organic material plays a significant role in improving the performance of organic device, such as organic light emitting diode, organic solar cell and organic thin film transistor. In this paper, we employ the space charge limited current (SCLC) method to evaluate the electron mobility of the controlled device based on tris (8-hydroxyquinolinato) aluminum (Alq3). The zero-field mobilities and field-dependent factors of the four devices are fitted respectively. The results show that depositing Al as top-electrode onto buffer layer LiF (1 nm) and Alq3 (100 nm) can significantly improve the the zero-field mobility and field-dependent factor of Alq3. The reason for that is that LiF could strengthen the complex reaction between Al and Alq3 to form Li+1Alq-1 particles, which leads to the enhanced ohmic injection and electron injection.