This research is for investigating the pre-engineered steel buildings according to deformation systems considering semi-rigid connection, which is considered as a correct model. Semi-rigid connection was proposed by Aleksander. K, in which the value of semi-rigid joint is dependent on connection types, material and section details. In this research, the deformation systems were analyzed using second order effects (P- effects) by Sap2000. P- effects and semi-rigid connection were simultaneously combined in evaluating the internal forces and displacement of the structure. In order to evaluate the accuracy of the current research method, the results were then compared with those designed by the current Vietnamese standard. The results show that the combination of P- effect and semi rigid connection have great effect in calculate the pre-engineered steel structures compared with other models.