In this study influence of different parameters included Quality of construction, life cycle of pipeline and data accuracy of previous in–line inspection tools for Determining In-Line inspection interval is presented. Affect of two parameters included quality of construction and life cycle of pipeline has investigated by reviewing history of several gas transmission pipeline in comparing with bath tube diagram. An understanding of magnetism flux, and flux leakage is needed to understand the capabilities of MFL (magnetic flux leakage) inspection systems. Due to the uncertainties in both, load and resistance parameters, the deterministic methods are unable to estimate the failure probability of a corroded pipeline during pipeline service life. The reliability assessment of pipeline involves the estimation of failure pressure and evaluating of the limit state function. Modified B31G failure pressure model has used. A steady state corrosion rate is assumed to estimate the growth in dimensions of corrosion defects. The first order second moment iterative method is employed for carrying out reliability analysis. Determination of considered level of failure probability for inspection/ repairing programs is necessary. In this regard influence of accuracy for in–line inspection tools (magnetic flux leakage) data for achievement to determined level is shown.