This paper describes estimation results of thermophysical properties of stainless steel (SS) containing 5mass% boron carbide (B4C) in the solid phase. First, we synthesized SS-B4C samples with emphasis on 5 mass%-B4C and SS using a hot press method. Homogeneity of our produced SS-B4C sample was evaluated by chemical composition analysis, metal structure observation, and micro X-ray diffraction (XRD). Both the density and specific heat of the SS-B4C sample were evaluated. These measurements proved that the density in our sample was lowered and the temperature dependence of the density, along with the elevation of temperature, became gradual compared to that of grade type 316L stainless steel (SUS316L) used as a reactor material by addition of B4C. The specific heat became slightly higher than that of SUS316L by addition of B4C and showed similar temperature dependence up to 800°C.