A unified model coping with both physical and optical dot gains on color tone reproduction of print is proposed. The physical dot gain, Δσ, is approximated by a quadratic function of nominal dot percentages, σ0. The function, for each color, is characterized by a single parameter depending on printing technologies as well as ink-paper interactions, and has a symmetric form around σ0 = 50%. Applications to a color laser printer (on office copy paper) reproduces the experimental dot gain curves fairly well. Dependence on physical dot percentage, σ = σ0+Δσ, results in the optical dot gain and in turn the overall dot gain asymmetric forms, plotted against σ0. Furthermore, theoretical analysis reveals fundamental differences between physical and optical dot gains.