Abstract: A practical yet simplified method for P- effects analysis of tall building steel frames with semi-rigid connections is presented. By taking partial frame as an analytical subassembly element and using a rotation spring which has variable bending stiffness to model semi-rigid of beam-column connection, this paper derives a simplified non-iterative inner force and lateral deflection computation formula taking into account P- effects for tall building steel frames with semi-rigid connections. The numerical examples show that semi-rigid of beam-column connections indeed decreases the lateral shear stiffness of steel frames and obviously increases the horizontal displacement and P- effect on the steel frame. Therefore, the influences of semi-rigid connection should been considered in P- effects analysis of tall building steel frames. The presented method is not only relatively simple, practical and easily applicable to manual computation but also has a rather high accuracy, and can be used by the engineers in preliminary design for tall building steel frames with semi-rigid connections. The main effect factors of P- effects in a tall building steel frame with semi-rigid connections are the column shear stiffness, the proportion of the beam liner stiffness to column liner stiffness as well as the different form of semi-rigid connections and connection rigid coefficient .