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The alloying contents with the same Zn/Ca atomic ratio of 0.13 on the corrosion behavior of Mg‐Zn‐Ca (ZX) alloys were investigated. The second phases in ZX10, ZX20, ZX30, and ZX51 alloys were identical with a majority of Mg2Ca and very few Ca2Mg6Zn3 and their distribution changed from isolated to continuous state in ZX30 and ZX51. ZX10 had the lowest corrosion rate of 0.55 mm/year after 168 h immersion in simulated body fluid solution due to the smallest number of sites of galvanic corrosion. The corrosion rate of ZX30 alloy was the highest (0.72 mm/year), while though ZX51 alloy had the most Mg2Ca, its corrosion rate was reduced to 0.69 mm/year, resulting from the small size and thin Mg2Ca. In situ microstructure observation demonstrated the high fraction Mg2Ca phase and continuous Mg2Ca with bulky size were the main reason for the fast corrosion rate. [ABSTRACT FROM AUTHOR]