Abstarct: The construction of precast as well as cast in place concrete structures have come to the forefront of the industry practice due to their fast construction and low costs. In recent decades a new system of construction consists of reinforced concrete columns and steel beams (RCS) has been considered by many researchers and engineers. In fact charactristics such as deformability and energy dissapiation of steel beams and stiffness of concrete columns are improved in RCS system. In addition, the time of construction in RCS system, in comparison with concrete frxame, is reduced. The use of steel beams makes it possible to construct larger spans and as a result, reduces the number of columns. One of the main challenges of RCS system is the connection of steel beam to concrete column. In this research, the seismic behavior of RCS connections is investigated by incorporating various parameters such as concrete compressive strength, thicknesses of joint steel plates such as Through Plate (TP), Side Plates (SP) Face Plates (FP). This evaluation is accomplished by numerical analyses under cyclic and monotonic loading in different beam / column bending capacity ratio. For this purpose, two RCS connections were modeled under cyclic loading in ABAQUS finite element software and their results were validated by exprimental results. Then, the effect of different parameters such as: the contribution of the triple resistance mechanisms in transferring the total connection moment, the thickness of the steel plates in joint region, the compressive strength of the concrete, on the design and seismic behavior of RCS connections was thoroughly investigated. The results showed that: 1. The major contribution of total connection moment belongs to the TP mechanism that increases with increasing the thickness. The minimum and maximum contribution of TP mechanism in the total moment transfer of the connection from the beam to the concrete column is 75% and 86%, respectively. 2. By increasing the ratio of bending capacity of the concrete column to steel beam, the contribution of the TP mechanism to the two other mechanisms increases and vice versa. 3. The increase in the thickness of the Face Plates (FP), which has the load transfer function to the Side Plate (SP) mechanism and the concrete diagonal strut mechanism (DS), increases the load transffering contribution of the two mechanisms. 4. Increasing the compressive strength of the concrete and the thickness of the side plates (SP), both reduce the load transferring contribution of the side plate mechanism (SP) and the concrete diagonal strut mechanism (DS). 5. If the ratio of the concrete column bending to the steel beam is less than 0.98, the failure mode is transferred from the steel beam to the through plate (TP) of the joint region, and if the ratio of the bending capacity is increased from 1.37, the contribution of the triple joint resistance mechanism of the connection will not change. Therefore, the optimum interval for using the connection with the ratio of the column-to-beam moment capacity is 1.77 ≥ β ≥ 0.98.