Abstract: Based on stress-strength interference model, calculation formula of reliability and mean safety coefficients and their relationships are derived, corresponding to different distributions of stress and strength. Through practical calculations it is shown that, when the distributions of stress and strength are subordinated to normal distribution or others, the value of mean safety coefficients is about the same, though the value of reliability is quite different. It is safer for common components to use normal distribution, and it is more exact for very important components to use logarithmic normal distribution or Weibull distribution.