Reinforced concrete (RC) structures in a marine environment deteriorate with timedue to chloride-induced corrosion of reinforcing bars. This paper presents reliability-based durability assessment of existing RC structure in a marine environment. A simple assessment criterion with partial factors that satisfy the target reliability level within the remaining life-time of existing RC structure is presented. A procedure to obtain the partial factors updated by the Sequential Monte Carlo Simulation is indicated. In this procedure, the chloride concentration distribution by coring test is used as observational data. Partial factors could be determined by taking into consideration the reduction of uncertainties associated with the prediction of chloride-induced corrosion. Partial factors for durability assessment of existing RC structures could be reduced compared with thosefor durability design of new RC structures.