Small single-electron devices (SEDs) consisting of many Si nanodots are fabricated on a silicon-on-insulator (SOI) wafer by means of pattern-dependent oxidation (PADOX) method. We investigated SEDs from two kinds of viewpoint. One is how to fabricate the nanodots, especially coupled nanodots, which are important to achieve quantum computers and single-electron transfer devices. The other is demonstration of new applications that tolerate the size fluctuation. In order to achieve multi-coupled nanodots, we developed an easy method by applying PADOX to a specially designed Si nanowire which has small constrictions at the ends of the wire. We confirmed the double-dot formation and position of the Si nanodots in the wire by analyzing the measured electrical characteristics. To achieve high functionality together with low-power consumption and tolerance to size fluctuation, we developed nanodot array device which has many input gates and outputs terminals. The fabricated three-input and two-output nanodot device actually provide high functionality such as a half adder and a full adder.