We deposited (Mn,Zn,Fe)1−xO thin films of a wüstite structure on SiO2/Si(100) by ion beam sputtering using a single-crystal Mn-Zn ferrite target. The wüstite structure of the as-deposited film, confirmed by XRD, TEM, and XPS analysis, appeared to originate from an oxygen-deficit ambient and also from the preferential resputtering of the oxygen ions in films during deposition. The as-deposited films showed ferrimagnetic characteristics having quite a large Ms in spite of their crystallographic structure, wüstite. Such an unusual phenomenon is presumably due to the different magnetic moments of the constituent cations with disordered distribution. This wüstite phase could be transformed into the spinel ferrite phase with the same preferred orientation during postannealing under an appropriate oxygen partial pressure. The interplanar distance of the as-deposited films decreased with increasing Ts due to a release of compressive stress. The Ms of the film had a maximum value at about 275 °C, while the resistivity, mainly governed by the grain boundaries, was almost the same irrespective of Ts.