This paper presents the deflection behavior of reinforced concrete slab strengthened with carbon fiber reinforced polymer (CFRP) plates to increase service load. The tested specimens represent the reinforced concrete flat slab which always have problem with large deflection after long term use. CFRP strengthening was the famous technique for solving this problem. Four slab specimens without and with CFRP plates were tested in order to investigate the ability of slab deflection reduction. The tested result show that the strengthening method by CFRP plate can significantly reduce the slab deflection. The deflection was reduced about 38 and 50 percent for slab strengthened with 1 CFRP plate and 2 CFRP plates, respectively. Furthermore, the equation of the effective moment of inertia from ACI440.1R 06 considering CFRP section in the calculation of moment inertia of crack section has been applied to predict the deflection of slab strengthened with CFRP plate. It was found that the given effective moment of inertia can be used to predict the slab deflection very well compared to the experimental results.