A quantitative phase-field model is developed to study the evolution of vacancy cluster in Fe. Total energy of the system is constructed based on the assumption of ideal gas state equation, and an approach to linking the computational parameters in the phase-field model to the experimental properties of Fe is provided. Such a phase filed model is employed to quantitatively investigate the nucleations, growths, and coalescences of voids in single and polycrystalline Fe. The effects of grain boundary on voids evolution are also investigated. These results provide a way of further studying the evolution behaviors of both H/He gas atoms and voids in Fe.