Ni-Zn ferrite (100) thin films were deposited to investigate the effects of low-target-voltage sputtering using an electron cyclotron resonance (ECR) sputtering apparatus. Saturation magnetization equivalent to that for bulk Ni-Zn ferrite could be obtained at any target voltage. Full width at half-maximum of the rocking curves decreased with decreasing the target voltage in the range of −400 to −100 V. When the target voltage was −50 V, Ni-Zn ferrite thin films showed relatively high coercivity and broad rocking curves. The reasons were considered to be the influence of impurity gas and unsuitable oxygen gas flow rate. The validity of low-target-voltage sputtering was confirmed for the reactive ECR sputtering method.