A MOS transistor includes a pair of impurity regions formed in a substrate as spaced apart from each other, and a gate electrode formed on a region of the substrate located between the pair of impurity regions. Each of the impurity regions is formed of a first epitaxial layer, a second epitaxial layer on the first epitaxial layer, and a third epitaxial layer on the second epitaxial layer. The first epitaxial layer is formed of at least one first sub-epitaxial layer and a respective second sub-epitaxial layer stacked on each first sub-epitaxial layer. An impurity concentration of the first sub-epitaxial layer is less than that of the second sub-epitaxial layer.