Abstract: Aim Proposes a new method of determinating the coefficient of offset in the planetary gear train having a small tooth difference. Methods The interfe- rence condition Gs is made to approach the expected critical value e under which there is no interference. Results A formula is established to determine the optimal coefficient of offset. Conclusion When the coefficient of offset is used to design modified gears, the contact ratio, is increased besides a reduction in gear size, and no interference will occur.