The effects of liquid viscosity and tube length on flooding are not considerd in the conventional correlations. In this experiment, the viscosity effects are studied experimentally for the vertical circular tube. In order to analyze the viscosity effects, aqueous glycerol solution with the range of 0.98$\sim$115cp are used for working fluid. Measurements are carried out by using a capacitance film meter for void fraction, DP cells for pressure, and rotameters for the flow rate. Several test section geometries are investigated by changing the air-water mixer ad the tube length. According to the observation, many roll waves are generated on the surface of liquid film and amplitude grows as the waves move downwards along the test tube. The roll waves become unstable and breaked into entrainment as the air flow rate increases to a certain value. The effect of tube length and that of air-water mixer are explained by the wave celerity. Prediction of flooding point in tube side of a steam generator for the case of small break loss of coolant accident (SB-LOCA) shows that Wallis`` prediction overestimates it at high liquid flow rate regime.