Properties of the weld joints of a bake-hardening steel (BH steel) for automobile were investigated. In the first experiment, the BH steel was plastically strained followed by Nd:YAG laser welding or TIG welding and subsequent heat-treatment. Then the influence of laser welding or TIG welding on behavior of bake-hardening was investigated. In laser welding, the heat-affected zone (HAZ) was narrow and the welded joint fractured in the base metal. In TIG welding, the large softening zone existed in the HAZ, because the plastic strain was released by the heat input of welding. So the weld joint fractured in the HAZ. In the second experiment, the BH steel was welded with laser welding followed by plastic deformation and subsequent heat-treatment. Then the influence of laser welding on behavior of bake-hardening was investigated. The heat treatment resulted in the increment of hardness in both the base metal and weld metal by bake-hardening. The increment of hardness by bake-hardening (ĢHVBH) in the weld metal was larger (31.1HV) than in the base metal (18.2HV). This is caused by the difference of the amount of dissolved nitrogen and carbon. The change in the tensile strength of each region corresponded with the change in hardness. Thus, the linear relation between hardness and tensile strength was obtained. It is, therefore, possible to quantitative prediction of the tensile strength from the hardness. As for the effect of plastic strain on bake-hardening, the amount of bake-hardening reached a maximum value at the plastic strain of 5% or more.