Multilayered aluminum and titanium base composites reinforced with 18%Ni maraging steel sheets were fabricated by the single-shot explosive welding technique. Microstructures and tensile strength of these composites were examined. The effect of heat treatment on the tensile strength of the composites is analyzed based on the rule of mixtures. When the reinforcement exhibits the maximum strength by aging, the ultimate tensile strength of both composites aged after welding is about 1.5 times higher than that of the aged one before welding. The former is slightly higher than that predicted from the rule of mixtures. This increment is attributable to the strengthening by the thin intermetallic compound layer formed by aging. The effect of the thickness of the intermetallic compound layer on the tensile strength of the titanium base composite is also described.