The <i>in-situ</i> measurement of phase stress under tensile deformation on an A6061 alloy reinforced with SiC whiskers (MMC) was performed using the X-ray diffraction technique. Phase stress in Al matrix was increased linearly up to 2800×10<SUP>–6</SUP> in strain and then saturated immediately. On the other hand phase stress in SiC whiskers showed an unstable stress behavior. It was decreased at first because of the Poisson effect from Al matrix but reversed over 500×10<SUP>–6</SUP> applied strain. The measured phase stress behavior in elastic region agreed with the calculations using micromechanics based on Eshelby/Mori-Tanaka model except for this unstable internal stress region. Facets were observed on the fatigue fracture surface and many clusters of the whisker existed beneath the fracture surface. These suggest a clustering deformation in the inside of MMC. Incidentally, each phase in the MMC under tensile deformation cause a strain level difference that arise from the difference of elastic constant between Al matrix and SiC phase. This quantity of plastic strain mismatch at the interface, Δ<I>ε</I><SUP>p</SUP> was estimated by using micromechanics. The Δ<I>ε</I><SUP>p</SUP> change under tensile deformation reflected internal fracture such as formation of micro voids or exfoliation of whiskers occurred at the interface.