Product/metal ratio (PMR) was introduced as a novel criterion for the evaluation of electrolytes on micro-arc oxidation (MAO) of Mg and its alloys. The criterion initially sprang from Pilling-Bedworth ratio (PBR), focused on the roles of electrolytes for the compactness of the fabricated coatings, and took attention on properties of reactants/products during MAO. Meanwhile, based on our experiments as well as the results from literatures, the effects of electrolyte additives on morphologies and com-positions of the fabricated MAO coatings of Mg alloys were exploited for verification and supplement of the initial criterion. In combination of the initial PMR criterion and experimental verification, PMR could be represented by special mode (PMRs=Voxide products/Valloy substrates) and general mode (PMRg= PMRs+ PMRd). The ideal PMRs should be between 1 and 2, while PMRd is related to the coating deposition during MAO. PMRd is a supplement to PMRs when the effect of the overlaying prop-erty (O) of the coatings and the effective deposition (D) of electrolyte composites are considered (PMRd=f(O, D). O is related to the melting point (MP) and boiling point (BP) of the MAO products. D is related to the effective reactions between alloy substrates and electrolytes during MAO. Product / metal ratio (PMR) was introduced as a novel criterion for the evaluation of electrolytes on micro-arc oxidation (MAO) of Mg and its alloys. The criterion initially sprang from Pilling-Bedworth ratio (PBR), focused on the roles of electrolytes for the compactness of the fabricated coatings, and took attention on properties of reactants / products during MAO. Meanwhile, based on our experiments as well as the results from literatures, the effects of electrolyte additives on morphologies and com-positions of the fabricated MAO coatings of Mg alloys were exploited for verification and supplement of the initial criterion. In combination of the initial PMR criterion and experimental verification, PMR could be represented by special mode (PMRs = Voxide products / Valloy substrates) and general mode (PMRg = PMRs + PMRd The ideal PMRs should be between 1 and 2, while PMRd is related to the coating deposition during MAO. PMRd is a supplement to PMRs when the effect of the overlaying prop-erty (O) of the coatings and the effective deposition (D) of electrolyte composites are considered (PMRd = f (O, D). O is related to the melting point is related to the effective reactions between alloy substrates and electrolytes during MAO.