The formation process of Z-type hexagonal ferrite, Ba Co Fe O , prepared by polymerized complex method and the effects of pH control of citrate complex solution were investigated. By heating a precursor, a pyrolyzed complex polymer, spinel phase, M-type ferrite and Y-type ferrite were formed sequently with increasing the heating temperature. By heating up to 1200°C, monophasic Z-type ferrite was not synthesized in usual preparation procedures. So pH of citrate complex solution was changed and the effects on formation process were examined. As ammonia solution was added to change the complex solution from acidic to basic, the phases in the pyrolysate changed form barium carbonate to barium nitrate and the crystallinity of the constituent phases decreased. By controlling pH to 11, monophasic Z-type ferrite was obtained by heating at 1200°C. On the other hand, pH control of the complex solution toward basic condition suppressed the formation of Y-type ferrite. It was revealed that the optimum conditions for preparing the complex solution depended on phases even for same Ba-Co-Fe-O system. 3 2 24 41