An experimental investigation is carried out to study the characteristics of flow boiling oscillation in a vertical minichannel with an inner diameter of 2.15 mm. The working fluid is deionized water at a constant system pressure of 0.101 MPa. The characteristics of flow oscillation with different working conditions are analyzed in this paper. With other constant working conditions, it is found that the oscillation periods of the pressure drop and the wall temperature decrease when the inlet subcooling decreases, while the fluctuation amplitude of pressure drop oscillations changes a little. The effects of heat flux and mass flux on the flow boiling oscillation are discussed with the equilibrium vapor quality (x(e)). Meanwhile, the x(e) which is at the onset of flow oscillation is affected by the mass flux. Finally, a new model which is based on the flow instability ratio (FIR) is proposed to predict the onset of boiling a two-phase flow oscillation, and a good agreement is achieved when comparing with the experimental results.