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.