Abstract: Addressing the fact that as a common case,flaws,even openings are developed with the(Φ 6 mm×0.6 mm)small-sized light-wall stainless steel pipe for aviation purpose,when it is undergoing the expanding process(to get its OD enlarged up to 9.5 mm),the expansion-caused flaws and the processing steps which cause the gradual increase of the hardness of the pipe are analyzed by means of the optical microscopy,the tensile testing machine and the hardness tester. The result of the study identifies that the said flaws normally stay with stainless steel pipes having rather high hardness and small grains,and that the hardness of the pipe is increasing linearly,when its OD is getting smaller along with the drawing operation during the processing.