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From the viewpoint of applying electric resistance welded (ERW) steel tubes for automobile structural parts, the formability degradation of steel sheets in a series of forming steps is investigated. Sheets are subjected several forming steps, such as a roll-forming into tubes, pre-forming, bending and bulge-forming. The formability degradation highly depends on the microstructure and aging property. Deterioration of the formability of sheets is diminished by the existence of the second phase in the ferrite matrix. The formability degradation of the steel sheet having an aging index (A.I.) ≤10MPa is well restrained so that the tube shows a much higher elongation and expanding ratio on bulge forming than the tube having A.I.>10MPa. These phenomena are applicable to all the tubes of tensile strength level form 290MPa to over 590MPa.