Friction dampers have low cost and good performance. Therefore they are in a good position on passive control devices. There are performance indexes to optimize the slip load of friction dampers. Also investigators created a simplified method to determine the optimal slip load of Pall friction dampers. Rotational friction damper was introduced at 2000. Experimental and numerical tests were done on this damper. In this paper, steel frames with rotational friction dampers, and variable slip loads, are investigated, by 540 nonlinear dynamic time history analysis. These analyses were done on three frames with different heights and different accelerograms. Also slip load is obtained from the simplified method. These results are achieved, that the different performance indexes of damper, give a nearly response to determine the slip load. Performance of dampers is better by increasing the heights of structure. And the error of simplified method to determine the optimal slip load of rotational friction damper less than 2 percent, in comparison with exact method.