Surface roughness significantly affects the work efficiency and life of machine parts that interact with each other. There are many parameters that affect surface roughness, such as the material being processed, cutting tool, cutting parameters and cooling shape. For this reason, it is very important to optimize the processing parameters and to introduce the appropriate processing conditions. In this study, surface roughness was investigated according to the spindle speed and feed per tooth parameters in the turning of the 4340 tempering steel, and the optimum values of processing conditions were determined. The Taguchi experimental design method was used to save time and cost. Experiments were based on the Taguchi L16 orthogonal array, and signal/noise ratios were used in the evaluation of the test results. Optimum surface roughness values were determined with Taguchi optimization. Analysis of variance was also performed. Confirmation tests were carried out to verify the work and it was seen that surface roughness optimization was successfully carried out in the turning of the 4340 tempering steel.