Abstract: The elastic analysis of composite beams is an important content in bridge engineering. However, an accurate calculation of stresses of composite beams with elastic shear connections (with slips) is quite inconvenient, because a closed solution is needed. Getting a closed solution is possible for simple loads and is very difficult for complicated loads or statically indeterminate beams. At present a simple and accurate calculation method is not available and a related content cannot be found in (national norm) “code for the design of steel structures”. For this reason, a new method called “effective rigidity method” for calculating stresses of composite beams is proposed. The method is simple, accurate, with a clear mechanic bases and suitable for arbitrary loads and rigidity of shear connections. A composite factor obtained from calculation results shows how many composite effects are and can be also used to calculate flexural rigidity.