Abstract: A semi-analytical method is provided to predict the behavior of beam-to-column semi-rigid connections of steel structures. A calculation formula of beam-to-column connection rigidity is derived firstly based on the theoretical analysis of semi-rigid connections. With the derived theoretical formula of connection rigidity and a series of experimentally measured values of loads and displacements at the end of beam, an effective semi-analytical method for the prediction of semi-rigid connection rigidity of steel structures is presented. The proposed method can eliminate the difficulties and the accuracy errors in directly measuring the joint rotation angles in the traditional experimental methods, and can predict more actual performance of semi-rigid connections than the finite element method. An example is given to show the simplicity and the accuracy of the semi-analytical method presented for obtaining the M- θ curves of beam-to-column semi-rigid connections of steel structures.