Abstract An accurate prediction of crack shape development during fatigue propagation plays a key role in structural reliability assessment. To predict crack shape, aspect ratio variability (a/c; a: crack depth, c: half crack length) is wildly used and different correlations to estimate this later exist in literature. In this paper, reliability index of a pipeline with axially surface crack is evaluated. Initial aspect ratio was varied. Tow dimensional crack growth models based on Paris Law coupled to liner elastic fracture mechanics were used. The objective is to clarify the effect of initial aspect ratio and it variability during propagation on the life time prediction of cracked pipeline. Based on Importance sampling, reliability index is plotted against number of load cycle. Results show an important role of initial aspect ratio and crack growth model in reliability index estimation. Calculation of different sensitivity factors illustrates the effect of uncertainty and correlation of Paris law parameters on the predicted life time of the pipeline.