Plastic gears have many merits ; silent running, self-lubrication, corrosion-proof, etc ; but they have many demerits too ; lower load-carrying capacity, lower-thermal resistance, etc. Because the plastic is expensive, the plastic gears should be designed as small as possible. Then, plastic gears which have smaller module, pressure angle of 14.5°, contact ratio above two and helical trace are proposed in order to increase economically their load capacities. The load capacity of a gear pair whose bending strengths are balanced by applying thick teeth to the plastic gear and thin teeth to the steel gear is double that of the usual gear pair having the same thick teeth.