The surface roughness model was developed in terms of main cutting parameters such as cutting speed, feed rate and depth of cut, using response surface methodology. Machining tests were carried out in turning AISI 1050 hardened steels by cubic boron nitride (CBN) cutting tools under different conditions. The model predicting equations for surface roughness of R-a, R-z, and R-max were developed using an experimental data when machining steels. The results indicate that the feed rate was found out to be dominant factor on the surface roughness, but it decreased with decreasing cutting speed, feed rate, depth of cut for these tools. In addition, average surface finish of Ra value produced by CBN cutting tool was about 0.823 mu m when machining hardened steels. However, the Ra value decreased about 0.55 mu m in terms of trial conditions. Moreover, a good agreement between the predicted and experimental surface roughness was observed within reasonable limit. (C) 2007 Elsevier Ltd. All rights reserved.