Abstract: This paper is concerned with the initial stiffness of semi-rigid connections under linear assumption. The fixed-end moments of semi-rigid beams under concentrated, uniform and linearly varying loads are obtained. The influence of semi-rigid connections on the internal forces of steel frame is discussed. It is shown that the initial stiffness of semi-rigid connections is mainly related to the bending stiffness of the joints, thickness and location of bolts. The flexibility of the connections affects the behavior of the frame. It can decrease the negative end moment of beams and increase the positive moment at the mid-span of beams. Thus, it is unreasonable to design under rigid connection assumption. It is concluded that the semi-rigid connections should be taken into account in the analysis and design of steel frames.