Unlike terrestrial communications, unmanned aerial vehicle (UAV) communications have some advantages such as line-of-sight (LoS) environment and flexible mobility, but the interference will be still inevitable. In this paper, we analyze the effect of interference on the UAV communications by considering the LoS probability and different channel fadings for LoS and non-line-of-sight (NLoS) links, affected by the elevation angle of communication link. We then derive a closed-form outage probability in the presence of interfering node for all the possible scenarios and environments of main and interference links. After discussing the impacts of transmitting and interfering node parameters on the outage probability, we show the existence of the optimal height of UAV that minimizes the outage probability. We also show NLoS environment can be better than LoS environment if the average received power of interference is more dominant than that of transmitting signal in UAV communications.