During recent years, the flat slab and flat plate building construction have become popular in Myanmar. These slab systems have many benefits which enhance speeding up construction, low building height and economical. In this study, the 12½ storeyed RC building with flat slab system and flat plate system have been analysed by ETABS. Flat slab and flat plate are designed by SAFE software. Both structures are situated in seismic zone 2B. The purpose of the research is to compare the structural behaviour of both structure such as base shear, story drift and story displacement. Moreover, another purpose is to present the difference between slab stresses and slab design. Load considerations are based on UBC-97 and structural elements are designed according to ACI 318-08.For these 12½ storeyed RC building, the comparison results show that the flat plate building is more beneficial than flat slab building. As the structural behaviour of flat plate building is better stiffness than flat slab building, flat plate building is safer. Moreover, flat plate building is more economical so steel area of flat plate building is less than flat slab building.