Optimal profile of knife, in which all points of the profile exit the soil at the same place during tillage, was determined for the following design parameters of the rotary tiller: radius (R = 0.25 m) and angular velocity ( w = 16.038 s -1 ), and for soil tillage parameters: working depth (a = 0.1 m) and working speed (v m = 0.77 m/s). The positions of the knife tip and every point of the knife were determined by parametric equations. First, the time interval in which knife tip reaches the critical point (soil surface) was determined. Also, the time intervals in which all points on the knife defined as X and Y with respect to the knife tip reach their critical point were determined. Equation which can describe optimal shape of knife profile can be obtained by equalizing the travelled distance in the x-axis direction of the knife tip with any other point on the knife profile. The curve, which represents the optimal shape of knife profile, was determined for the given parameters. If X is 30 mm, then Y is 6.024 mm, and if X is 60 mm then Y is 16.103 mm. The angle of inclination of the function Y(X) in the coordinate beginning (at the point of the knife tip) is 7,84°.