The three-dimensional (3D) integration process is a promising candidate to enhance electron-device performance. Typical 3D integration systems consist of vertically stacked several thin IC chips that are electrically connected with lots of through-Si vias (TSVs) and metal microbumps. Metal microbumps are surrounded by organic adhesive. An epoxy-based material, so-called underfill, has been widely used to fill the gap between several chips. In general, the coefficient of thermal expansion (CTE) of the underfill material is larger than that of metal microbumps. This CTE mismatch induces local bending stress in thinned IC chips. This local bending stress would affect the CMOS circuit in thinned IC chips. Therefore, we should suppress the local bending stress to realize 3D IC with high reliability. To suppress the local bending stress, we have proposed a novel underfill with negative-Thermal-expansion material. In this study, we investigated the characteristics of the negativethermal-expansion material surrounded by the matrix of the underfill.