Abstract: The double involute gear with ladder shape of tooth is a new type of involute gear which absorbs the advantages of both involute helical gears and double circular-arc gears. After the total contact ratio and the line of contact length are analyzed and researched on the double involute gear，the precise formulae are given. The worst and the best condition of combination for transverse contact ratio and overlap ratio are proposed. It is very important to design and analyze the transmission of involute helical gear and the double involute gear.

Key words: Contact ratio, Involute gear, Line of contact