The marine vessel reversing main gear tooth is subjected to three different loading cycles: ahead travel with load pulsing from zero to 100% of full power; astern travel with load pulsing from zero to about minus 66% of full power; and reversal of direction with load changing from 100% of full power to about minus 66% of full power. The number of repetitions of these three different loading cycles varies with the vessel duty cycle and life. The published values of allowable design stress for teeth are based on pulsing loads@ which must be modified for this third loading cycle. The tooth may also be subjected to mean stress due to shrink fitting of the gear onto a hub. Publications which address these conditions include: SGuide values for mean stress influence factor@ Ym@ of ISO 6336-3 gives a factor@ which derates the pulsing@ i.e. unidirectional@ allowable stress value for non-pulsing load. SThe American Bureau of Shipping Rules@ derate the allowable unidirectional bending strength by 10% for the reversing main gear tooth. (Idler gear teeth@ which are under bidirectional loading at full power@ are derated by 30%.) SDet Norske Veritas DNV Classification Notes No. 41.2 addresses gears: with other working conditions than pure pulsations; with periodical changes of rotational direction; and with shrink fitting stresses. For gears with occasional full load in reversed direction such as the main wheel in a reversing gearbox@ the derating factor of 10% is recommended. This paper evaluates the derating factor for marine reversing main gear tooth allowable bending stress using the Goodman fatigue line and Miners equation as a function of the average number of changes in vessel direction per hour@ shrink fitting stress values@ and different materials based on the AGMA values for allowable stress and life factor.