Composite steel-concrete columns have an adequate seismic performance because of their stiffness, strength and ductility and have been found to be cost effective especially for multistory buildings under horizontal load. However, inelastic response of composite columns is significantly affected by the connection of column bases. Particularly, the concentration of inelastic demand occurring is on the anchorage bolts and relies chiefly on the bond type mechanisms. This analytical research paper investigates the effect of anchor bolt diameters on the inelastic performance of partially encased steel concrete composite column under base plate connection. The base connection and its foundation system were designed in compliance with the rules provided by EURO code. Several simulations under monotonic lateral loads (pushover tests) were performed. Finally, it is found the results of numerical simulation are evident for the current design of base connection for partially encased steel-concrete composite column is failed due to anchor bolt fractures (concentrations of inelastic demand occur in the anchorage bolts). In addition, through the modification of anchor bolt diameter, the ductility demand of the structural system is transformed from anchorage bolts in to the steel base plate. Therefore, the lateral force resisted by the structural system and the global deformation is improved; it indicates, there is utilized base plate capacity not being used. By increasing the total area of the anchorage bolts up to the average value of 27.77% beyond the current design results, an average lateral force of 12.89% and an average lateral drift of 18.67% improvement are attained by the structural system.