Abstract: Bridge structures are at risk from aging, leading to structural deterioration from service demands, from increasing traffic and heavier loads, from aggressive environmental attack and other physical mechanisms. The loading capacity and safety level of existing bridges are of world-wide concern. This paper introduced briefly the structural reliability theory, which is currently a powerful approach for condition assessment of existing bridges, and presented the difference between new bridge design and existing bridge assessment in the framework of reliability theory. By comparing Chinese code with American Code, the paper pointed out: 1) the load capacity of existing bridges is evaluated based on the in-site measured strength & dimension, system effect and the increased uncertainty due to the development of deterioration; 2) the safety condition of existing bridges is assessed multi-levelly according to the measured Average Daily Truck Traffic (ADTT); are two points in which American Code is more advanced than Chinese Code. The loading history of existing bridges is a kind of proof test, which may increase effectively the evaluated load capacity of bridges, and avoid the unnecessary posting or strengthening. At the end, the paper proposed the modification of a live-load factor according to ADTT and the modification of load capacity of existing bridges according to the loading history.