Abstract: The microstructure characteristics of martensite/austenite(M/A) islands of four kinds of X80 and X100 steel pipelines were investigated by optical microscopy and SEM contrastively, and the influences of M/A island microstructure characteristics on mechanical properties were studied combining with the tests of mechanical properties. The results show that the M/A islands presented the forms of bulk or spot with sharp corners and unevenly distributed in the microstructure of X80 pipeline steel. M/A islands of the X100 pipeline steel were spot or strip and dispersively distributed. The effects of volume fraction, average dimension, form and distribution of the M/A islands on strength, yield ratio and toughness were distinct. When the volume fraction of M/A islands was 1799% and the islands distributed uniformly with small and dotted or crossed strip forms, the pipeline steel exhibited excellent strength-toughness matching.