Background:Studies have shown that the flexibility of the connections has an important role in the distribution of forces and moments in the structure. This also applies for the displacements, deformations and the stability of the structure. Objective:The objective of the present study is to investigate the influence of the stiffness of column web-panel and connection on the ultimate response of Moment Resisting Frames (MRFs). Methods:A comprehensive parametric study was carried out. In particular, a set of one hundred and twenty planar frames was analyzed, considering three approaches for modelling the joints. Results:The results highlight that neglecting the influence of the connection and column web panel leads to significant over-estimation of the global instability factor α cr , which can lead to unconservative design and assessment of steel frames, especially for those cases subjected to severe horizontal forces as the seismic actions. Conclusion:Accounting for the joint deformability in MRFs is important even for the cases with connection stiffness (k b ) larger than 25 times the beam stiffness where EN1993-1-8 allows neglecting the model of the connection stiffness. Indeed, the comparison of cases with k b ≥ 25 between the models with and without the connection deformability show that of α cr can decrease from 5% to 16%, depending on the refinement of the modelling assumptions. This decrease also lead to higher second order effects and thus to higher design forces but also the overall stiffness of the frame is overestimated.