In the API X65 steel, effects of rolling and cooling conditions on microstructure and mechanical properties were studied. In the case of accelerated cooling after multi-pass rolling in the high/low unrecrystallized range, the tensile strength was 574-670 MPa and the impact toughness was 74-109 J. In the case of accelerated cooled to 550°C and then interrupted by air cooling, on the other hand, those values were 524-538 MPa and 100-135 J, respectively. Whereas the former exhibited the continuous yielding, the latter showed discontinuous yielding. In addition, yield ratio increased from 0.59-0.67 to 0.85-0.87, accompanied with the enhancement of yield strength. Ultrafine ferrite grains formed by the strain induced dynamic transformation during the severe rolling and second phases formed during cooling were observed. In accelerated cooling and interrupted cooling conditions, main second phases formed after cooling were martensite and pearlite, respectively. Separation cracking mostly observed at interfaces of ferrite matrix and second phases, may be attributed to the intrinsic interfacial weakness.