The impact-wrenching is known as one of the most practical methods for the tightening of bolted joints, but few studies had been made on the process of increasing of clamping force by the successive impacts. This study is made on the clamping force of bolts wrenched by the rotary hammer which gives successive impacts to tighten the bolted joints. The clamping force given by successive n impacts is proportional to √<n>・F_1, where F_1 is the initial clamping force. The process of the tightening of bolted joints is analysis and the theoretical results are compared with the experimental data. The experimental results agree with the theoretical values.