This letter considers the transmission design in unmanned aerial vehicle (UAV) assisted cloud radio access network (C-RAN). We aim to maximize data rate of the system by optimizing user equipment (UE) association, UAV placement, UEs' and UAVs' transmit power. We take UAVs as flying radio remote heads (RRHs) to serve UEs on the ground and maximize the sum rate of all the UEs. In order to solve this non-convex problem, we propose an iterative algorithm based on the successive convex approximation (SCA) and alternating iterative methods. The numerical results show that the proposed algorithm can effectively maximize the sum rate of UEs in the UAV assisted C-RAN system.