Abstract: A method for calculating the modification coefficients of strength reduction factors for MDOF effect is developed.With the proposed method the modification coefficients of strength reduction factors for MDOF effect are investigated on four different RC frame structures,which were strictly designed according to the Chinese seismic code and subjected to a great number of earthquake ground motions.The statistical analysis result shows that the modification coefficients can be accurately estimated by a simplified expression and the effects of various factors on the modification coefficients can be evaluated.It is concluded that the modification coefficients are primarily affected by the ductility level and periods of structures and they will increase apparently with an increase in the ductility level and structural period.It is also shown that the site conditions only have small effect on the modification coefficients.