Based　on　the　nutation　movement　of　a　rotating　coin　on　a　table,　the　formulae　for　calculating　the　transmission　ratio　between　two　nutation　gears　is　developed.　The　tooth　profile　with　a　double　circular　arc　for　helical　bevel　gears　in　nutation　drives　is　proposed　and　then　the　meshing　equation　during　the　nutation　process　and　the　universal　equation　of　the　crown　gear　tooth　surface　with　helical　double　circular　arc　profiles　are　obtained　based　on　the　gear　meshing　theory.　Further,　the　modeling　for　nutation　drives　with　the　proposed　profile　is　carried　out　in　a　virtual　working　process.