Modified tooth profiles and their deflections cause a gear pair to contact away from the theoretical line of action on which contact is assumed to occur in most present theories, so that they result in transmitting inconstant rotational motion. Actual points of contact which are not on the theoretical line of action can be described with the tangential-polar coordinates. With these coordinates and measured single-flank errors, gear rotational motion will be analyzed in this series of reports. In this first report, the engagement between modified tooth surfaces of a helical gear pair is replaced with that of equivalent tooth profiles with the same single-flank error. The equivalent profile is expressed algebraically through the tangential-polar coordinates and results in an involute curve with a base circle whose radius varies. Through the equivalent profile, a path of contact of the gear pair which transmits inconstant rotational motion is analyzed.