Abstract: Carbon migration process around the fusion boundary in the pearlite-austenite dissimilar steel welded joints was studied with “in-situ” mode by using FESEM and EDS.The carbon migration occurred obviously in the fusion boundary during post-weld heat treatment.The width of the martensite-like layer kept invariable during heat treatment period.However,carbon migration played different actions in various widths of martensite-like layers,i.e.,the migration rate in a wide martensite-like layer was faster than that in a narrow martensite-like layer.