The steel moment resisting frames are used as a one of the ductile systems for bearing earthquake lateral forces. The behavior of rigid beam-column connections has the major effect on the steel moment resisting frames.in order to provide required rotational capacity and ductility of connections. The typical connections which is offered in design codes such as Iranian code 10th were used. In some of the common executions in Iran, a special method know tree-column is used in which the increasing of the beam section from plastic area to column’s face in provided by tapered sections. This kind of connection is not a typical so it’s behavior hasn’t been already considered. In this dissertation the hysteresis behavior of these connection will be studied. In order to make sure of FEM’s results, hysteresis experiments on real samples will be done. Then, reforming the models of finite element, if required, more parametric studies to identify the behavior of connections will be done, as identifying of drawbacks. So the possible solutions for improving their performance will be considered. Due to increase of section’s height along the length of tapered section, the unstabilities of web is possible. Studying the possibility of web unstabilities and local bucklings, the capacity and ductility of this kind of connection, assuming the possible failure mechanisms, is considered using the plastic analysis and the theory of yield lines.