This paper presents a link reliability estimation method for road network reliability analysis. Link reliability is assumed to be the result of stochastic variations in link flow, and link reliability is defined as the probability that the demand flow on the link does not exceed the link capacity for a given time period. When link flow variation is identified as a probability density function, link reliability can be determined from demand flow and link capacity. Three OD flow variation patterns are introduced and the variance of link flow is analyzed through traffic assignment. Variance estimation model (VEM) of link flow and link reliability estimation method based on VEM are developed. Numerical operations is executed verifying that the method provides sound estimators.