Spinel zinc ferrites ZnFe_2O)4,prepared by co-precipitation method using the zinc nitrate Zn(NO_3)_2·6H_2O and ferric nitrate Fe(NO_3)_3·2H_2O as the raw materials,were characterized by the thermo gravimetric analysis(TG)and differential scanning calorimeter(DSC),X-ray diffraction (XRD)and scanning electron microscope(SEM).The influence of synthesis conditions,such as Zn/Fe molar ratio,pH value,the sintering temperature and time,on the microstructures was detailedly in-vestigated.The relationships between the microstructures and the synthesis conditions were discussed.The results show that the pure spinel zinc ferrites ZnFe_2O_4 are formed when the Zn/Fe molar ratio is 1.05:2 at pH＝8.5 or Zn/Fe molar ratio is 1:2 at Ph＝9-10,and the precursors are sintered at 1100℃ for 4 h.Especially no other phases are observed when the Zn/Fe molar ratio is 1:2 at pH＝10 and the precursor is sintered above 700 ℃for 4h.The higher sintering temperature and longer sintering time contribute to grain growth.