This study investigates experimentally the effect of power changing procedure, i.e., heating or cooling, on thermal hydraulic behavior of a double channel natural circulation loop. The data obtained at an inlet temperature of 60 degreesC and loop pressure of 1.5 bar (N-sub = 107) demonstrate clearly the existence of hysteresis effect at the onset of two-phase flows. During the heating procedure single-phase flow prevails in both channels while the heating power is increased gradually from 1.0 kW (N-pch = 1.25) to 4.5 kW (N-pch = 5.63). The loop Row rate, i.e., the sum of flow rate through each channel, increases with increase in power with an exponent of 0.336, which is consistent with theoretical prediction. The onset of significant two-phase flow takes place when the power for both channels reaches 5.0 kW (N-pch = 6.26), which agrees well with Saha and Zuber's model prediction on the net vapor generation. Moreover, the Row is found to be unstable. Nearly out-of-phase, large amplitude oscillations with reversed flow display in both channels. The two-phase flow and nearly out-of-phase oscillations persist while the power is gradually decreased from 5.0 kW (N-pch = 6.26) to 2.0 kW (N-pch = 2.50). The magnitude of oscillation is relatively independent of power, while the period is increased significantly as power is reduced. Single-phase flow is restored when the power is reduced to 1.5 kW (N-pch = 1.88) and the corresponding flow rate is close to that during the heating process. The hysteresis effect suggests that the loop state be a path function. This is of significant interest for the operation of a natural circulation loop, (C) 2000 Elsevier Science Ltd. All rights reserved.