In this paper, we propose a frontier-based online coverage path planner for 3D reconstruction with an unmanned aerial vehicle (UAV). The main purpose of this system is to explore the unknown map and simultaneously to make a 3D mesh map of the target structure. Most of the existing online coverage path planning method is sampling-based, which is inefficient. The proposed method tackles this problem by dividing the surface and spatial frontier. Then, the UAV focuses on the surface frontier to reduce unnecessary flight routes and time. The spatial frontier is only used for the collision-free path. The performance of the proposed method is verified in simulation.