A lot of bake-hardenable cold-rolled sheet steel has already been used in Japan. This type of steel is characterized by carbon atoms in solid solution, showing an increase of about 50MPa in yield strength through strain-aging effect during the paint-baking at 170degreesC after press-forming. However, bake-hardenable hot-rolled sheet steel has been far less studied. The author has studied bake-hardenable 600MPa (in tensile strength) grade hot-rolled sheet steel that is produced by a combination of a high nitrogen content and extra low temperature coiling practice, showing 70-80 MPa increase in tensile strength as well as in yield strength. This type of steel is named TS-BH steel because the steel exhibits an increase in tensile strength during the paint-baking, which is expected to be used for various components demanding fatigue properties. The effects of prestrain, nitrogen content, BH-temperature, and BH-time on the values of TS-BH and BH were investigated. Moreover, changes in electric resistance and in internal friction with strain-aging time were examined in order to clarify the TS-BH strengthening mechanism. Hasiguti peak of internal friction was observed for the steels with 70-90 MPa in TS-BH, which are strain-aged at 170degreesC for 20 min and over after 8% reduction skin-passed. It is concluded that TS-BH is caused mainly by precipitation of nitrides on the dislocations that were introduced by prestrain.