Abstarct: Following the Northridge earthquake of 1994, a significant number of steel moment resisting structural systems were damaged. Several studies have been conducted to improve the seismic performance of steel structures and specially their beam to column connections. One of the proposed connections for special moment resisting frxames included AISC-358 is ConXL moment connection. the collar systems of ConXL moment connection are precast steel elements. In this study the seismic behavior of ConXL moment connections with Iranian steel sections has been investigated. Also one of the Research purposes is identify optimum geometry of reduced beam section in ConXL moment connection. According to assumed criteria that includes: energy dissipation, distance between plastic hinge and panel zone, and preliminary stiffness of connection. Also the effect of filling concrete and pretension force in bolts has been investigated.