The calculation and analysis problem of a random factor influence degree in the gear reliability calculation is presented.The calculation method of gear reliability based on waybills distribution and ultimate status theory is used to calculate and analyze the change of a gear’s reliability.The reliability and reliability’s change value of cylindrical gear is calculated when the average value or variance of a random factor is gradually varied in the form of practical example.According to these calculation results

some random factors having a great influence upon gear reliability are found out.In order to ensure require of a gear reliability

these random factors are strictly controlled in the design and manufacture and using of a gear.These results will be for reference in the future gear reliability researches.