Abstract: A method for positioning seamed balls each having an endless curved seam line, comprising steps of: supporting each ball to locate its center at an origin of a three-dimensional orthogonal coordinate consisting of two horizontal axes X, Y and one vertical axis Z; rotating the ball on at least one of the axes X, Y until an optical sensor disposed on the axis Z specifies such a particular locus that intersects the seam line at four points during one full rotation of the ball to provide four arcs along the locus; finding the shortest arc among the four and locating its mid point on the axis Z by rotating the ball on the axis X; rotating the ball on the axis Y so that an intersection of a bisector and a part of the seam line is located on the axis Z; and rotating the ball on the axis Y until a hip center of the ball is located on the axis Z.