This paper is mainly concerned with the control system design for a quad-rotor UAV. The quad-rotor UAV is modeled by 6-DOF nonlinear equations of motion that includes rotor aerodynamics with blade element theory. Trim and stability analysis are performed to analyze and understand quad-rotor UAV dynamic model properties. Control system is designed based on PID autopilot and vision-based control technique. Additional vision-based control strategy is described for stabilization of the quad-rotor UAV using two on-board cameras in urban environments. Stabilization performances are assessed by 3D environment numerical simulation.