Abstract: The microstructures of X70 dissimilar weld joints were studied by optical microscope and SEM. Galvanic corrosion at different area ratios was investigated in simulated soil solution. The results indicated that the fusion zone was so narrow that no martensite-like structure was found. Fine dendrites were obtained in the weld zone, in which Nb and Mo seriously segregated in interdendritic. The corrosion resistance of the weld zone was excellent because of its passive film. There were coarse bainitic ferrite and nonuniformly distributed strip M-A constituent in the coarse-grained heat-affected zone (CGHAZ), and fine bainite existed in X70 base material. Their corrosion behaviors were not obviously different, but as the anode polarized potential exceeded to 150 mV the polarizability of CGHAZ was relatively high. The cathodic reaction rate was dependent on oxygen diffusion when the weld zone was coupled with X70 pipeline steel. The corrosion current density of X70 pipeline steel increased rapidly with increasing area ratio. The corrosion characteristic was similar to "Catchment Principle". Because of high cathodic potential the weld zone did not suffer corrosion.