A method of manufacturing a plurality of MOS transistors includes forming gate structures in first and second regions on a substrate and forming mask portions only between adjacent drain sides of the respective gate structures only in the first region. Dopant of a first conductivity type that is the same as that of the substrate, is implanted at first and second angles in both the first and second regions to form halo regions only in source sides under the gate structures in the first region and in both source and drain sides under the gate structures in the second region.