In this letter, we investigate the energy-efficient 3D placement of an unmanned aerial vehicle mounted base station (UAV-BS) that is capable of tilting its directional antenna. We attempt to find the optimal UAV-BS 3D location to support ground users, with minimum UAV-BS energy consumption. The 3D placement problem can be converted into a 2D placement problem without any loss of optimality, by obtaining the minimum altitude based on the elliptical characteristics produced by the tilted antenna. To solve the 2D placement problem, the gradient descent algorithm (GDA) is applied. Our simulation results show that the GDA performs very close to the 2D exhaustive search method, for finding the optimal UAV-BS placement. Furthermore, significant energy savings are achieved using the proposed antenna tilt method, compared to the benchmark schemes.