Various defects and cracks may be produced in the welding process in natural gas pipeline.Under variable pressure of natural gas,the cracks will be propagated it may lead to break and invalidity.The effect of internal surface cracks are more serious.It has been researched that the propagation along radial direction of internel surface cracks in the natural gas pipeline.The fatig life of pipeline has been calculated.On the other hand,considering the size of cracks, pressure of gas and character of material are random variable,and by using reliability method,the fatig reliable life of pipeline has been determined.This method has some practical value in design, working and overhaul of pipeline.