This is a simple analytical method for determining the general equation of absolute rotational speed of planetary gear trains. This method is based upon the general equation of motion of planetary gear trains (Krstich, 1985a, 1987b). The basic absolute rotational speeds for simple and compound planetary differential conical (bevel) and cylindrical (spur and helical) gear trains have been derived, and then the general equation of absolute rotational speed of planetary gear trains. This equation can be applied to any types of simple and complex arrangement of planetary gear trains by using only train values (gear ratios) of the planetary differential two–gear train or the planetary differential pair of gears (Krstich, 1985b).