Regarding the outlet condition of flow from thin-film evaporators, it was postulated that the liquid fillet adjusted the width at the exit so that the energy of flow was minimum at a section about 15 times the thickness of liquid fillet upstream from the exit. By comparing a series of experimental data with theoretical calculation, it was demonstrated that this postulate was good for liquid of viscosity less than 6mm2/s but not for liquid of viscosity higher than 17mm2/s. Based on the hypothesis, the exit width of liquid fillet is predicted, and the overall pattern of liquid fillet in an evaporator can be calculated using the predicted exit width as the initial condition. Predicted flow pattern in the entire evaporator was compared with experimental data at different operating conditions, and good agreement was demonstrated in all cases except near the inlet zone where the flow pattern was disturbed by the incoming condition of fluid.