The authors found that there is an interference which causes unfavourable contact at the tooth-top edge of gears with a concave tooth profile, and established the design method for eliminating the interference. Two dimensionless parameters to calculate the bending and twisting deflections of pinions with Z₁ = 3 to 5 were obtained by numerical calculations and experiments. Changes in the tooth bearings and gearing errors caused by elastic deflections were examined numerically. Load carrying capacity of Novikov gears with a small number of pinion teeth was investigated to clarify the effects of hardness upon the surface durability of the Novikov gears with Brinell hardnesses in the range of 190 to 330 HB. The best results were obtained when a low hardness steel was used for both the pinion and gear and also when a harder pinion was combined with a gear made of a low hardness steel.