Provided are a crystalline InZnO oxide semiconductor, a method of forming the same, and a semiconductor device including the crystalline InZnO oxide semiconductor. The crystalline InZnO oxide semiconductor includes an oxide including In and Zn, wherein in Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analysis, a content of In among In and Zn is about 30 at % or more and about 75 at % or less, and the crystalline InZnO oxide semiconductor has a peak showing crystallinity at a 2-theta value between about 32.3 degrees and about 33.3 degrees in X-ray diffraction (XRD) analysis.