In this letter, a novel Core-Substrate Gate-All-Around (CSGAA) nanowire structure has been proposed, investigated and simulated systematically based on 3D numerical simulation. This new structure is characterized by a Core-Substrate directly connected to the substrate wafer or well which is formed by a bulk region inside the tubular channel. Comparisons are carried out between conventional Gate-All-Around (GAA) nanowire transistor and newly proposed CSGAA structure. This budding structure exhibits a maximum of four orders of magnitude of reduction in off-state current, high saturation current and low performance degradation. With the help of Core-Substrate, the CSGAA structure is able to maintain excellent performance despite geometric and structural variation. Its special structure is very suitable for the use of vertical Gate-All-Around nanowire structure, making it a promising candidate of future high performance and low power CMOS devices. INDEX TERMS CMOS, core-substrate gate-all-around, CSGAA, field effect transistor, GAA, nanowire.