A mathematical model accounting for the pressure oscillation when flooding occurs is presented to predict the velocity limit of an immiscible vapor mixture condensing in a vertical tube. This model incorporated the parameters of vapor inlet velocity, pressure drop, interfacial friction stress, tube inside diameter, and the fluid physical properties. The in-tube condensation experiments of an oil-steam vapor mixture with various mass ratios have been conducted using an 18 mm i.d. brass tube and a 20 mm i.d. glass tube, respectively. The model predicted that data agreed very well with the experimental data, indicating that this model is valid to predict the velocity limit of the immiscible vapor mixture condensing in a vertical tube.