Abstract: The causes of errors were analyzed when the outer diameter of gears and cushion plate for rotary kiln were measured to use π ruler in directly, and the control and verification methods for eliminating measurement errors were proposed. The results showed that the errors were bigger which were measured directly for the outer diameter of the shim plate and the diameter of the pinion tooth top circle using π ruler, and the measured values were less than the actual size. The measurement errors must be considered when the π ruler was used to measure directly the outer diameter of the pad plate and the diameter of the pinion tip circle compensation. The measurement errors compensation value could be calculated according to relevant theoretical dimensions, and the corresponding operation verification should be performed.