Cutting parameters optimisation studies in milling are usually concerning the main machining parameters as cutting speed, feed rate and depth of cut. The step-over ratio is hardly ever considered in optimisation of milling process. The main aim of this study is to discover the impact degree of cutting parameters, especially step-over ratio, on surface quality, and also to realise a satisfactory optimisation with considering it. Accordingly, sample workpieces of AISI 1113 were subjected to end milling at high spindle speeds by using a TiAlN coated flat end mill without using coolant in a CNC vertical machining centre. According to the surface roughness results, the optimum cutting parameters providing the minimum roughness were designated by using the Taguchi method. ANOVA (Analysis of variance) was applied to find the impact degree of parameters on surface quality as statistical way. The results showed that spindle speed, step-over ratio, feed rate and depth of cut affected surface roughness by 32.34, 28.92, 12.38 and 4.02%, respectively. The step-over ratio has a significant effect in formation of surface roughness almost equal to spindle speed. The average surface roughness was successfully improved up to 76.7% with the optimised machining parameters with considering step-over.