The mechanical properties and deformation behavior of each constituent layer of multilayered steel composites were examined using microtensile testing. Three-layered integrated steels consisting of SUS420 and SPCC (cold-reduced carbon steel sheets) were fabricated by a cold-rolling process. Different heat treatment processes were used to prepare three types of specimens (as-rolled, 823K-2 min heat-treated, and 823K-500 min heat-treated), and the effect of heat treatment on their mechanical properties was investigated. In the as-rolled specimens, the average tensile strengths in the SUS420 and SPCC layers were 1063 and 606 MPa, respectively, while in the specimens heat-treated for 500 min, they were 680 and 451 MPa, respectively. The tensile strength decreased with the increase in the heat treatment time. The tensile strength of the specimens was also calculated by using the rule of mixture. For the as-rolled specimens and the 823K-2 min heat-treated specimens, the calculated value was consistent with the measured value; however, for the 823K-500 min heat-treated specimens, the calculated value was lower than the measured value. This result suggests that the necking of this layered structure was effectively obstructed by the outer ductile layer. The micromechanical characterization technique used in this study is useful not only for investigating deformation behavior but also for designing multilayered steel composites with superior mechanical properties.