This study develops a novel method of detecting a parking space and generating time-varying target lines of an automatic parking system according to image processing technique. The aim of this research is to detect the parking frame drawn on the floor by a robot equipped with a single camera and produce the image information, the slope and intercept of the target line for the control system. The time-varying target lines to be followed by the robot are generated by using Hough transform from a captured image, together with thresholding and Canny edge detector. The proposed method is evaluated by some experimental results.