The paradigm of smart cities links the industry of telecommunications directly to sustainable economic growth and high quality of life. To meet this trend, this article develops a 5G IoT network with UAVs for future smart city architecture. We particularly emphasize IoT in the sky in a bid to unify the global world via heterogeneous smart devices through 3D connectivity. In our design, UAVs form a 5G hierarchical IoT network in the sky by connecting to a number of base stations on the ground. Simulation results reveal that the leader UAV has a significant impact on the performance of the whole system, and the proposed approach has an obvious advantage over the existing ones.