Small-sized UAVs are increasing more and more rapidly in urban circumstance in recent years. However, the traditional path planning methods always threaten simplification, and fail to solve the problem of path-planning in the circumstance efficiently Therefore, cell is introduced to define the flying space, digital cellular map and path cost function are defined. At the same time by using ant colony algorithm this paper plans the path for UAVs, thus avoiding any kinds of irregular obstacles. The simulation results show that the proposed method can avoid obstacles logically under condition of the obstacle model without simplification. By doing so, this plans a safe flyable path in urban building areas for UAVs.