The invention provides a semiconductor device and a manufacturing method thereof. The semiconductor device is capable of providing a groove MOS transistor having upwards-drain electrode, and the semiconductor device could comprise a first conductive trap on a substrate of the semiconductor, a second conductive trap on the first one, a groove formed by removing part of the first conductive trap on the second conductive trap; a grid in the groove, with dielectric substance thereof located between each grid and the groove wall; a first conductive source electrode region around a transverse surface of the grid, and a second conductive source electrode region located on the second conductive trap; a common drain electrode located between the grids, connected to the first conductive trap. The invention is capable of forming a groove MOS transistor having low output resistance and high frequency characteristic with low cost, obtaining low output resistance with small area, and is capable of integrating the semiconductor device with high grade.