Deformed cladding in nuclear power plants can partially block the path of coolant flow and thus alter the heat transfer characteristics between the cladding surface and coolant. In this research, critical heat flux(CHF) in a sudden expansion flow path, was experimentally investigated. Flow boiling CHF experiments were carried out under atmospheric pressure with deionized water for vertical upward flow, with four expansion ratio cases investigated with mass flux ranging from 50 to 250 kg/m(2) s. CHF was detected at two different locations. In experiments with either a low expansion ratio(epsilon < 1.5) or low mass flux(< 150 kg/m(2) s), CHF occurred at the test section outlet with values similar to results from a plain tube with the same inlet diameter. On the other hand, in experiments with a high expansion ratio(epsilon > 1.5) and high mass flux(> 150 kg/m(2) s), CHF was detected at the sudden expansion point. Quality at the sudden expansion point was almost zero, which implies that nucleated bubbles under the effect of sudden expansion flow induced CHF at the sudden expansion point. The reason for this transition of CHF location was suggested that local bubble stagnation induced by a balance between drag and buoyancy forces on the bubbles resulted in local flooding in the downward flow.