Abstract: Hierarchical planning structure is adopted in the control system of inspection robot for power transmission lines.A production reasoning system is used on the behavior hierarchy for on-line planning to produce the behavior sequence.Combined with the interpretation knowledge base which is produced with off-line planning method,the behavior sequence is interpreted to engender motion sequence which is used to execute the automatic control for the robot to cross obstacles. Control flow sheet of the planning process and the planning method is discussed in this paper,and the feasibility of the motion planning method is validated by the experimental results.