In this study, the effect of surface roughness of bolted joint members on the friction behaviour of the joint was investigated by considering theoretical studies carried out previously. For this purpose, AISI 366 structural steel sheet was used as the sample material. After the contact areas of the samples were defined and polished to different surface qualities, the surface asperities were recorded and the samples were bolted under different compressional moments and then subjected to a torsional moment on a torsion machine. The effects of surface roughness, compressional moment and torsional angle on the torsional moment were determined. It was observed that the surface roughness plays a major role in the joint reliability. High R(a) values of the interfaces lead to easy loosening of the joint. Therefore, improvement of the surface quality of mechanical joints is very useful for tightening the joints.