Axial void fraction profiles in subcooled flow boiling in a vertical annulus were obtained using gamma attenuation techniques for different levels of mass flux, wall heat Aux and inlet subcooling. A mechanistic model was developed to predict the axial void fraction profile. Various closure relationships were used and a new model, for dividing the wall heat flux between vapour generation and heating the subcooled liquid, was incorporated. The predictions of the proposed model were found to be in good agreement with the present experimental data, as well as other available data, on axial void Fraction distribution in low pressure subcooled flow boiling. The proposed mechanistic model does not require prior identification of the location of the onset of significant voids.