Abstract: Theory foundation of crack evaluation for oil & gas pipelines is fracture mechanics.In linear elasticity fracture mechanics. SIF is an important index to judge whether the cracks of oil & gas pipelines get into unstable progress state or not.Oil & gas pipelines are made from metal material, which is basically impossible to produce"strict"linear elasticity fracture, however, if the size of plastic zone is much Iess than that of crack, adopting linear elasticity theory to analyze the produced error will be acceptahie after proper correction.When the size of plastic zone and the size of crack are in the same magnitude, linear elasticity theory is needed including COD criteria and J integral.In this case, stress strength factor, failure assessment block diagram and finite element method can be used to evaluate cracking state of oil & gas pipelines.Finite element method can cover the shortage of another 2 methods in applicability, but it has shortages in modeling and computing efficiency; and it is difficult to carry out a preventive evaluation.