The part coordinates are always calibrated before the arc welding robot carries out some tasks. Traditional methods use expensive and comp licated devices for measuring. A novel method to quickly calibrate the part coordinateswas p resented, using only the internal coder data of robot and coordinates transform calculation to get the coordinates transform matrix between the part frame and the basic frame of robot. It was easy to operate and its calibration result was precise. The method was imbedded in the self-developed off-line programming software for MOTO-MAN SK6 arc welding robot. Based on the algorithm and off-line programming, robot writing was realized. Experimental results showed the validity of the calibration method.