Robot trajectory generation for spray painting have highly demand in modern manufacturing. One of the most common surfaces in spray painting operations is ruled surfaces. In this paper, a geometry based spray paint trajectory generation system for ruled surfaces has been developed to simulate the spray painting process using articulated arm robot. The developed system uses the geometry information of both the required ruled surface to be painted and the paint spot to generate spray painting trajectory for Labvolt RoboCIM 5150 articulated robot. The developed system is designed by using Matlab software, and has been simulated and evaluated using the Labvolt system. The results have shown that the generated trajectory achieves satisfactory performance.