The effect of whisker reinforcement on the precipitation behavior of SiC whisker reinforced Al-Cu-Mg composites has been investigated by the differential scanning calorimetry, transmission electron microscopy, and macrohardness measurement. Through the thermal analysis on the composite and matrix was not changed by whisker reinforcement. However, the result of macro-hardness measurement showed that the age-hardening of whisker reinforced composites was generally accelerated. Based on a simplified model of whisker reinforced composite and by using the finite element method, a umnerical calculation for the misfit strain generated by the solid solution treatment was conducted. Through in situ observation of the precipitation process in AI-Cu-Mg matrix by a transmission electron microscope, the role of increased dislocation density by a large misfit strain in whisker reinforced composite has been investigated. A possible mechanism of accelerated agehardening in whisker reinforced Al-Cu-Mg composites has been discussed in view of misfit strain and increased dislocation density