Microstructural phase of linepipe steels depends on different isothermal conditions. Thermal cycling testing of X90 linepipe steel was conducted using a thermomechanical simulator. The results have shown that with increased holding time, the microstructure constituents change from the martensite and bainitic ferrites to granular bainite and polygonal ferrite. There was the amount of martensite in the microstructure for isothermal holding times of 5 and 10 s at 700ºC. The effects of the martensite strengthening were weak. The changes of the hardness curve are decided by microstructural phase and the precipitation behavior. The interphase precipitation seems to begin for isothermal holding times greater than 30 s. It has a peak value for isothermal holding time of 600 s. Some fcc (Ti, Nb) (N, C) particles which belong to the MX-type precipitates were obtained at this condition. It is a major microstructural contributor to the hardness. The beneficial effects of the precipitates decreased considerably due to coarsening of the precipitates with prolonged holding, which results in the hardness quickly dropping. As a result, the hardness exhibited a low value after isothermal holding for 3600 s because the extent of precipitation strengthening was lessened and a lot of polygonal ferrites were formed.