Technical Abstract: Horizontal axis rotary tillers can cause irregularities in soil tilling depth when they are not used in appropriate operational and constructional conditions. In such cases, mixing and breaking of soil through tilling depth will not be in the same degree and some untilled ridges will remain on the bottom of the tilled layer. Available equations and methods are not practically adequate for determining how much the height of ridges is affected by design and operational characteristics of these tillers. In this study, a method based on deriving geometrical relationships between the affecting parameters was developed to determine heights of ridges occurring at the bottom of the tilled layer. The ridge heights can be calculated for various combinations of blade peripheral speed, forward speed, rotor radius, and the number of blades on one side of a rotary tiller flange. This method allows investigation of effects of operational and constructional properties of horizontal axis rotary tillers on tilling depth. This analysis shows that the heights of ridges on the bottom of the tilled layer increase with increases in forward speed and decrease with increases in blade peripheral speed, rotor radius, and the number of blades on one side of a flange.