Infilled steel frame structures are known to perform better as compared to infilled concrete frames under seismic loading. In comparison with masonry infilled reinforced concrete (RC) frame structures, very little research has been done on masonry infilled steel frames structures. Contribution of masonry infill in the positive performance of the infilled structures is widely recognized but no codes provide the clear and detailed guidelines on the design of infilled frame structures especially infilled steel frame structures due to the complex and unpredictable behaviour of masonry during an earthquake event. This study aims to provide a detailed insight into the composite behaviour of steel frame structures with unreinforced masonry infill under seismic loading. Time history analyses have been performed on masonry infilled steel moment resisting frame structures and structural performance has been evaluated in terms of global structural performance parameters of fundamental time period, maximum roof displacement and base shear and local parameters of interstorey drift ratios, structural member forces and infill stresses. The effect of masonry infill on the performance of infilled steel moment resisting frames has been investigated by varying the number of bays, number of storeys, percentage opening in the infill wall, location of the opening, type of openings, number of openings, infill strength, outer frame strength and infill thickness. The results of this research will help to understand the complex behaviour of masonry infilled steel frames for different variations mentioned above.