Abstract: Correlation has an important effect on structural system reliability and it is widely accepted that the increase in correlation is benefit to the reliability of parallel systems and is harmful to that of series systems. A mathematical explanation for this conclusion is given based on Hohenbichler's and Johnson's formulas. The effects of correlation on series system and parallel system in both aspects of benefit and harm are proved respectively and the degrees of effect are quantified. Based on this result, the effect of correlation on the series-parallal mixed system and the degree of its effect are studied. And thus this study result will be helpful to deal with the correlation and correlative data treatment more effectively, To compute the failure probability and reliability index more accurately and improve the computational efficiency.