Abstract Structural, AC and DC magnetic properties of polycrystalline Zn 1− x Co x Fe 2O 4 ( x=0.2, 0.4) samples sintered at various temperatures (1100–1300 °C), and various dwell times (0.2–15 h) have been investigated thoroughly. The bulk density of the Zn 0.60Co 0.40Fe 2O 4 samples increases as the sintering temperature ( T s ) increases from 1100 to 1250 °C, and above 1250 °C the bulk density decreases slightly. The Zn 0.80Co 0.20Fe 2O 4 samples show similar behavior of changes to that of Zn 0.60Co 0.40Fe 2O 4 samples except that the bulk density is found to be highest at 1200 °C. The DC magnetization as a function of temperature curves show that the Zn 0.60Co 0.40Fe 2O 4 sample is ferrimagnetic at room temperature while the Zn 0.80Co 0.20Fe 2O 4 sample is paramagnetic at room temperature. The T c of Zn 0.80Co 0.20Fe 2O 4 sample is found to be 170 K from DC magnetization measurement. Separate measurement (AC magnetization), initial permeability as a function of temperature shows that the T c of the Zn 0.60Co 0.40Fe 2O 4 sample is 353 K. Slight variation of T c is observed depending on sintering condition. The initial permeability for the Zn 0.60Co 0.40Fe 2O 4 composition sintered at 1250 °C is found to be maximum.