In this work, we investigate the potential of using unmanned aerial vehicles (UAVs) in supporting ultra- reliable and low-latency communications (URLLC). The key idea is to leverage better link qualities provided by high possibility of line-of-sight (LoS) links in UAV communication systems. We first characterize the latency, the reliability and the network availability of UAV communication systems. We show that the probability of the existing of LoS links and the network availability are strictly concave in terms of the distance between the user and the UAV. Then, given the density of UAVs, we optimize the altitude of UAV and bandwidth allocation for minimizing the required total bandwidth of URLLC. Numerical results show that increasing the density of UAVs can remarkably reduce the total bandwidth required by URLLC in suburban areas. It is also shown that, in urban areas, using a single ground-to-air wireless link is not sufficient for ensuring the QoS and the network availability of URLLC, implying the needs of deploying multiple ground-to-air or ground-to-ground wireless links in order to support URLLC.