This paper presents a study of the Taguchi design methodology, which is applied to optimize flank milling operation parameters when machining titanium alloy Ti-6Al-4V in conventional and high speed regimes. This study includes cutting speed, feed rate and depth of cut. Experimental runs are conducted using an orthogonal array of L9(33), with measurement of surface roughness. ANOVA analyses are carried out to analyze the effect of these operation parameters, and the optimal parameters combination is determined by seeking the best surface roughness. The analysis of result shows that the optimal combination for good surface roughness is high cutting speed, low feed rate. Finally, confirmation run verifies the results, which indicated that Taguchi method is both efficient and effective in determining the Ti-6Al-4V flank milling parameters for the optimization with minimum number of trials.