To develop strain sensors with excellent reliability, stability, sensitivity, reversibility, and facile processing still suffers great challenge. In this work, the glass micro-beads (mu-GB) with high modulus were added into poly (ethylene-co-1-octene)/multi-walled carbon nanotubes (POE/MWCNTs) composites to construct modulus mismatching domains, which generated asymmetric deformation in the composites during compression and then improved piezoresistive sensitivity. The mu-GBs with diameter of 45, 75, and 105 mu m were used to evaluate the size effect of modulus mismatching domains on the piezoresistive sensitivity of the composites. It was very interesting to find that the composites with different diameter of mu-GBs exhibited different piezoresistive sensitivity. The highest piezoresistive sensitivity was found in the composites with the smallest size of mu-GBs (45 pm). Additionally, the piezoresistive sensitivity was also improved by increasing the content of mu-GBs. For example, the gauge factor of the POE/MWCNTs composites increased from 1.9 to 6.4 by the incorporation of 50 wt% mu-GBs with 45 mu m in diameter. The mechanism of the asymmetric deformation in the composites with modulus mismatching domains under cyclic loading was discussed in detail. Furthermore, the strain sensor with mu-GBs had excellent reliability, stability and reversibility, which was attached to a sport shoe and exhibited a good cyclic signal response during walking. Besides, the Young's modulus and compression modulus were also enhanced by the addition of mu-GBs. For example, the Young's modulus of the POE/MWCNTs composites showed 300% enhancement by adding 50 wt% mu-GBs with 45 mu m in diameter.