Abstract: The formation mechanism, processing-affection factors of M-A island and effects of ratio, size, quantity, distribution of M-A island on mechanical properties of the X100 pipeline steel were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy and image analysis software(Image-plus 6.0). The results show that the ratio, size and quantity of M-A island increasd with the increase of cooling rate in the range of 10 - 60 ℃·s-1; the size of M-A island decreased with increasing the final cooling temperature in the range of 350-500 ℃, well ratio and quantity increased at first and then declined. Both strength and toughness were related to shape and distribution of M-A island.The steel in which M-A island dispersed distribution in small size had excellent performance on both strength and toughness.