In this study, the effect of inlet subcooling on sustained and transient boiling flow instabilities induced by forced convection in a heated single channel upflow system has been experimentally investigated by using Freon-113 and Freon-11 as the test fluids. Two major modes of oscillations, i.e., density-wave and pressure-drop types have been identified. Stable and unstable regions have been determined in system pressure-drop versus mass flow rate planes for various values of inlet fluid temperature. Stability and instability boundaries at each constant heat flux are obtained on a mass flow rate versus inlet subcooling plane. Transient instabilities resulting from a step increase in power input and a step decrease in flow rate at various values of inlet subcooling have also been investigated.