In this thesis, we address the energy-efficient 3D placement of unmanned aerial vehicle mounted base station (UAV-BS) that is capable of tilting adjustably its directional antenna. In other words, we attempt to find the optimal UAV-BS 3D location to support ground users (GUs), with minimum UAV-BS total energy consumption (TEC). The 3D placement problem is converted into a 2D placement problem without any loss of optimality, by obtaining the minimum altitude based on the elliptical wireless coverage characteristics formed on the ground by the tilted antenna. We apply the gradient descent algorithm (GDA) to minimize the TEC of the UAV-BS, in order to solve the 2D placement problem. Simulation results show that the GDA performs very close to the 2D exhaustive search method, for finding the optimal UAV-BS placement, and the optimal UAV-BS 3D placement is variant depending on the throughput requirement and GU density. We also verify that our proposed antenna tilt method achieves significant TEC savings compared to the vertical UAV-BS service case without antenna tilting, and fixed antenna tilt angle cases. Furthermore, when single or multiple UAV-BSs are operated according to the increase in the size of the region in which GUs are distributed, we provide the insight and guideline for UAV-BS operation considering the UAV-BS energy capacity by verifying energy-efficient 3D placement, and analyzing the system TEC.