A path tracking control approach based on line of sight guidance is proposed for underactuated unmanned underwater vehicle（UUV）.Based on the line of sight guidance approach

error model for tracking is established.The position error control can be transformed into the combination of speed control

heading control and pitch control.Hence

the 6 degree of freedom（DoF）output space is simplified into 3 DoF output space.The controllers are designed respectively for the reduced 3 DoF motion

and the stability of the controllers are proved formally.A simulation is held to track a straight line which parallels the x axis

and the results prove the effectiveness of the approach.