Full scale sub-sample fatigue test of the mechanical components is direct relationship with the equalizing value of fatigue limit and the logarithmic standard deviation of the part. So, the full scale sub-sample fatigue test of the mechanical component plays a critical role on the assessment of the fatigue life. In this paper, the author implemented the standard deviation for the initial test data of full scale sub-samples with an epicyclic gear as example based on working standard of steeliness mechanical part. The test results declared that there were rather larger dispersion coefficients of standard deviation which influenced the safety fatigue limit of mechanical component due to a larger system errors were presence in the process of initial test. Compared to the previous test results, the safety fatigue limit of the steeliness epicyclic gear had been developed significantly by retest with reduced system errors.