Abstarct: One of the most popular seismic resisting systems is resisting moment frxame. In Iran and for this common method, using of flange plates is useful for connection of beams to column. Box columns are used in this research due to the common use of these type columns in Iran. At first a numerical model is generated in ABAQUS which has been verified by the results of experimental cases. This model has been applied for the parametric study of the dimensions of top and bottom flange plates. The results of numerical analysis has been studied and justified. The results are: The plastic rotation of the beam has been increased due to the enlargement of the length of the plates and it will lead to a drop in hysteretic loop. Using of the transverse weld will increase the performance of the connection. By reducing the width of bottom flange plates at its end, and increasing the width of bottom flange plates and also using a plate in center of panel zone, the probability of rupture at the ends of bottom flange plates will be reduced.