In this study, dynamics of a fixed-pitch co-axial rotor unmanned helicopter is modeled, and fundamental motion analysis is performed using derived model. The unmanned helicopter is considered as a rigid body system consists of 2 rigid bodies, and multi-body dynamics modeling technique named velocity transformation method is used for deriving the equation of motion. Force and torque terms applied to the helicopter are obtained by using aerodynamics of co-axial rotors including aerodynamic interference between upper and lower rotor. Finally, the motion analysis using the model is performed to establish the relation between the motion and the mechanical specification of fixed-pitch co-axial rotor unmanned helicopter.