The main goal of this paper is to investigate an onshore buried pipeline failure probability based on Magnetic Flux Leakage (MFL) inspections data. Degradation of an underground pipeline during its service life leads to reduction of the pipe wall thickness. Periodic in-line inspections are performed by onshore pipelines operators to detect corrosion anomalies and size their depth and width. Det Norske Veritas DNV-RP-F-101 analytical method of burst pressure calculation for a straight pipeline was applied. Criteria and formulation of a limit state function were presented to determine the burst pressure and corresponding failure probability of a pipeline with a large number of single part-wall defects. The paper illustrates pipeline reliability-based maintenance planning, in the case when a number of defects and its statistical distributions are known from MFL in-line inspection. The Monte Carlo numerical method was selected for estimation of pipeline failure probability due to the external corrosion and fluid pressure loading with respect to statistical distribution of input parameters were examined in this paper. A probabilistic methodology was applied to evaluate the part-wall external corrosion defects and their growth with time on gas transmission pipeline. It was assumed that failure probability of an underground pipeline is influenced only by the growth of the existing defects, whereas generation of new defects is neglected. A code-based engineering approach to estimate the failure pressure is selected as appropriate to be applied directly after in-line inspections, due to the scope of the available data, before any expansive field excavations. The results of this study shall help maintenance engineers solve the problems of an effective strategy in reliability-based high pressure gas pipelines management.