Ni-Zn ferrites are magnetic ceramics that are widely used in electric and electronic components. Among Ni1-xZnxFe2O4 ferrites, Ni0.3Zn0.7Fe2O4 is known to be the one of highest magnetic permeability in lower frequencies. This work is divided into two parts: the first part deals with the influence of sintering temperature and forming conditions on the microstructure of Ni0.3Zn0.7Fe2O4 ferrite. The second part deals with the influence of sintering temperature and environmental variations of temperature on the complex magnetic permeability of such ferrite. The ferrite studied in this work was fabricated by means of the conventional ceramic method. Complex magnetic permeability between 100 kHz – 100 MHz is discussed in relation to sintering temperature. The influence of environmental variations in temperature (-40 oC to +50 oC) on the complex magnetic permeability of the ferrite sintered at 1300 oC is discussed.