In this paper, a study is presented on the milling process of C45 steel by face milling cutters. An experimental process was performed according to an experimental matrix constructed by the Box-Behnken method. At each experiment, three cutting parameters were changed, namely cutting speed, feed rate, and depth of cut. Surface roughness has been selected as a parameter to evaluate the milling process. Analysis of experimental results has determined the influence of cutting parameters as well as their interactions on surface roughness. Two regression models for surface roughness have been proposed. In which one model uses the Johnson transformation. These two models were used to predict the surface roughness and then compared with the experimental results. The results show that the model using the Johnson transformation has higher accuracy than the model not using the data transformation.