A new carbon family with a prism structure has been found by quantum chemical calculations. The prism-$C_20$ has a $D_10h$ structure with double-layered decagonal rings connected each other by ten vertical CC bonds. The CC bond lengths are 0.144 nm on the decagonal rings and 0.148 nm on the side faces. The diameter of the ring is 0.465 nm. The $prism-C_20$ is expected to be stable, since the lowest energy barrier from it was estimated to be 158.0 kJ mol$^−1$.