In order to curtail unnecessary energy consumption and improve energy efficiency, the cell on/off technique arises, especially in dense small cell deployments. However, the Transition Time between ON and OFF state deteriorates the energy efficiency. In this paper, we mainly discuss the optimization for energy efficiency considering the impact of Transition Time. We discover the on/off gain window, in which a small cell performing on/off achieves higher energy efficiency than staying always-on. In addition, the existence of the optimal energy efficiency for an on/off small cell is verified, as well as its uniqueness in a certain range. Moreover, a fitted formula is given to approximate the optimal energy efficiency. Finally, an energy efficient scheduling is proposed to ameliorate the performance of energy efficiency in practical networks.