This study is based on the optimization of the parameters that influence the computer numerical control (CNC) milling operation during the machining of polypropylene+60wt.% quarry dust composite. The input parameters studied are the cutting speed, the feed rate and the depth of cut. These input parameters were optimized using the Taguchi optimization technique with the output response taken into consideration was the surface roughness. An L9 orthogonal array (OA) was selected and formulated in a commercial software Minitab 19 based on three factors and three levels combination. The signal-to-noise (S/N) ratio was analysed to give a combination of values of the input parameters that produced optimum results for surface roughness. The analysis of variance (ANOVA) was then conducted to determine the significance and percentage contribution of each parameter. From the results, the optimum values obtained were cutting speed of 1000 rpm, feeding rate of 120 mm/min and depth of cut of either 0.5 mm or 0.8 mm. The cutting speed had the highest contribution towards the surface roughness at 81.98%, followed by the depth of cut at 7.43% and the feed rate having the least contribution at 3.69%.