A large number of bridges and buildings that are currently used, are made of slabs with opening. Some of these buildings and bridges have been severely damaged because of natural disasters such as earthquake, storm and other effects such as materials fatigue and alkaline and corrosive agents. Therefore, retrofitting and strengthening of these structures is essential and affordable. In this research, by applying experimental models, the flexural behavior of reinforced concrete slabs with opening strengthened with different FRP systems and steel sheets have been studied. Eight specimens of reinforced concrete slabs designed according to the Iranian code ABA with the dimensions of 150×45×20 cm were manufactured. In these slabs, the amount of tensile reinforcement was equal to 0.25b. Two openings across the length of each slab with 10 cm diameter were created. One slab was considered as reference (not strengthened) and seven other slabs were strengthened with CFRP laminates, GFRP sheets and steel sheets, and tested. Forces, strains and deflections were recorded during the tests. The results of this research show that the flexural strength increases about 43% in the strengthened slabs with FRP laminates, about 57% with CFRP sheets, 45% with GFRP sheets and 15% with steel sheets.