Abstract: A golf ball position marker has a thin, circular body that is sufficiently transparent to permit viewing 1) of a body-defined, disc-shaped, oil-filled chamber, 2) of body-carried indicia located adjacent the chamber, and 3) of a steel ball that is movably housed within the chamber and that cooperates with the indicia to provide an indication of the slope and break of a portion of a golf green on which the marker is placed. The movable ball and the indicia cooperate 1) to indicate that a portion of a golf green on which the marker is positioned is "level" if the ball remains centered within the chamber, 2) to indicate the direction of slope of an inclined green by the direction in which gravity causes the ball to move from the center of the chamber when the marker is placed on an inclined green, and 3) to indicate steepness of slope of an inclined green by the speed with which the ball moves from the center to one side of the chamber when the marker is placed on an inclined green.