Probability density function(PDF),phase space diagram,and mutual information function are utilized to analyze normal sinus rhythm(NSR) and atrial fibrillation(AF) R-R intervals.It is found that for NSR R-R intervals,PDF is not symmetrically distributed when time delay is small,while PDF satisfies Gauss distribution when time delay is big enough,demonstrating that the correlation between neighboring NSR R-R intervals is strong.For AF R-R intervals,PDF curves are of Gaussian distribution and the curves are nearly the same for different time delays,demonstrating that the correlation between neighboring AF R-R intervals is weak.The same conclusion is drawn by phase space diagram and mutual information method.The less correlation between RR intervals of AF than that of NSR is the reflection of AF pathological information,which may be developed as a new diagnostic method.