The optimization of the machining parameters on surface roughness by Taguchi method can reduce the trial and error type experiments by using a matrix design. In this study, L9(33) orthogonal array of Taguchi experiment is selected for three parameters (speed, feed, depth of cut) with three levels for optimizing the surface roughness in precision turning on an HE-100-CNC-PC CNC (Computerized Numerical Controlled) lathe. The material used in this experimental is the alloy of aluminum (AM-40, Material number ENAW-5083) having excellent corrosion resistance with good weldability and formability in the form of round bar having diameter of 20 mm dimension. It is machined according to Taguchi’s L9 orthogonal array. Turning operation experiments were carried out on a CNC lathe that provides the power to turn the work piece at a given rotational speed and to feed to the cutting tool at specified rate and depth of cut. Therefore three cutting parameters namely cutting speed, feed and depth of cut need to be optimized. The feasible space for the cutting parameters was defined by varying the turning speed in the range 1000-1600rpm, feed in the range 0.02-0.04mm/rev. and depth of cut from 0.25 mm to 0.35mm.