Fe3O4 magnentic colloids were produced by co-precipitating a Fe2+, Fe3+, and Zn2+ containing solution with a NaOH solution at 70°C. Analyses results indicated that the Zn content of the Fe3O4 colloids was always less than the solution's Zn content. The difference was found to be the smallest when the colloid was precipitated at pH=10. The as values of the Fe3O4 colloid significantly improved by adding an appropriate amount of Zn ions and precipitaing it at an optimum pH value, pH=10. The optimum Zn ion content was 6.18 at%, which was corresponded to Zn0 · 1054Fe0 · 8145Fe2O4. At this composition the colloid's σa value was 94 emu/g, and increased by more than 34% of pure Fe3O4 colloid's σ value. By theoretical calculation the σs should increase 27.8%.