Optimization of surface roughness has been one of the primary objectives in most of the machining operations. Poor control on the desired surface roughness generates non conforming parts and results in increase in cost and loss of productivity due to rework or scrap. Surface roughness value is a result of several process variables among which vibration is of great significant. In this study, experimentation was carried out to investigate the effect of vibrations, forced and self excited, on surface roughness. For forced vibration, variable used to represent machine tool’s condition was vibration amplitude. Input parameters used, besides vibration amplitude, were feed rate and insert nose radius. Cutting speed and depth of cut were kept constant. Based on Taguchi orthogonal array, a series of experimentation was designed and performed on AISI 1040 carbon steel bar at default and induced machine tool’s vibration amplitudes in external turning operation. Analysis of variance, ANOVA, revealed that vibration amplitude and feed rate had moderate effect on the surface roughness and insert nose radius had the highest significant effect on the surface roughness. It was also found that a machine tool with low vibration amplitude produced lower surface roughness. Insert with larger nose radius produced lower surface roughness at low feed rate. The optimum conditions obtained through Taguchi were found in reasonable agreerment to those obtained with full factorial design through Genetic Algorithm. Department of Industrial and Manufacturing Engineering Secondly, experiments related to self excited vibration in internal turning operation were selected. Taguchi method was employed to minimize the surface roughness by investigating the rake angle effect in boring performed on a CNC lathe. The control parameters included, in addition to rake angle, were insert nose radius, cutting speed, depth of cut, and feed rate. Slight tool wear was taken as a noise factor. Based on Taguchi Orthogonal Array L18, a series of experiments were designed and performed on AISI 1018 steel. It was found that tool with a high positive rake angle and smaller insert nose radius produced lower surface roughness value due to low cutting forces generated. It was also revealed that high feed rate and low cutting speed has produced the lowest surface roughness. Key words: turning; optimization; Taguchi; surface roughness; vibration amplitude, ANOVA