Abstarct: Due to fast and simple installation, high quality of welding and decreasing the percentage of welding fault, steel column-tree moment resisting frxames is preferred rather than other systems. Control of rigidity or flexibility in moving the plastic hinge away from beam-column connection is the other advantage of this system. Seismic behavior of this system is the most important issue and probable fault of welding, flange plates and connection bolts should be studied. Failures in structures in Northridge earthquake can confirm the importance of the study, as most damages are due to brittle failure in connection because of deficiency in calculation principles and implementation. In this thesis, executive faults such as imperfection of welding geometry, welding quality and the deficiency of flange in beam-stub connection are studied.