Surface roughness of the work gear after plunge shaving is primarily affected by the arrangement of the serrations. A mathematical model for the serration cutting edge of the plunge shaving cutter is proposed, taking into consideration the necessary hollow tooth form. On the basis of the proposed mathematical model, the influence of the serration's geometry and position of the plunge shaving cutter on the tooth surface roughness of shaved gear are studied. The shaving process is divided into two categories: up-cutting and down-cutting. The surface roughness of the work gear is improved if the cutting edge of the serrations is arranged for up-cutting as much as possible. Experiments were conducted to validate the up-cutting assumption.