Based on the tests of six reinforced concrete slabs strengthened with fibre reinforced plastic (FRP) sheets and one reference slab without the sheet, the influences of factors such as type of FRP sheets (CFRP and GFRP), amount of FRP sheets and strengthening scheme upon the crack resistant performance were presented and discussed in this paper. In the end, a calculation method was put forward, which can be used to calculate cracking moment and crack width in reinforced concrete slabs strengthened with FRP sheets.