Abstract: The effects of cooling temperature and cooling rate on the size and volume fraction of M-A islands in thermal simulation samples of high grade X100 pipeline steel were investigated by the microstructure analysis method. The results show that reducing the cooling temperature and increasing the cooling rate can refine the microstructure, decrease the volume fraction of M-A islands, and make the M-A islands from larger blocks and strips into smaller and dispersed ones. The morphology of M-A islands in X100 pipeline steel with different cooling rates after hot rolling was observed by transmission electron microscopy (TEM). It is found that the M-A islands consist of retained austenite and different sizes and orientations of martensite plates at low cooling rate. Microtwins in the martensite plates prove carbon diffusion and show that martensite in the M-A islands is twinned martensite. At high cooling speed, the M-A islands, which change into the acicular and flaky, weaken the ferrite lath interface and reduce the toughness of the pipeline steel. So in order to obtain small size and dispersive M-A islands, the cooling speed must be controlled in a certain range.