In order to reduce the on-resistance in vertical power transistors, backside thinning is required after device processing. However, it is difficult to thin an SiC wafer with a high yield rate by conventional mechanical machining because its high hardness and brittleness cause cracking and chipping during thinning. In this study, a small rectangular SiC sample was thinned by plasma chemical vaporization machining (PCVM), which is plasma etching using atmospheric-pressure plasma. As a result, the sample was successfully thinned to 40 m without any cracking or chipping. Furthermore, the surface roughness was improved after thinning, and the edge of the wafer became rounded automatically. Therefore, PCVM can be used as an effective method for thinning SiC wafers.