Abstract: By using group method, high frequency fatigue test was conducted on X70 steel pipeline girth welding joint with a copper backing and ceramic backing respectively when the stress ratio was 0.1(r=0.1), and the crack initiation of the two kinds of samples were compared in certain fatigue load and cycle times. The results indicated that the copper infiltration in X70 steel pipeline girth welding with copper backing had little influence on fatigue strength and that the S-N curves of the two samples were almost the same. Another finding was that, under the same fatigue load and cycle condition, the crack initiation had no significant difference with the fatigue crack starting at the back weld toe. Furthermore, the fatigue fracture was analyzed by SEM.