The effect of ultra fast cooling (UFC) during hot strip rolling on the precipitation behavior of cementite in carbon steels and its subsequent strengthening effect have been investigated by controlling the cooling temperatures for three carbon steels with 0.04%,0.17% and 0.5%C respectively. The results show that the refinement of ferrite grains and the reduction of pearlite lamellar spacing might mainly be responsible for the strengthening of the two steels containing 0.04%C and 0.5%C respectively, while no nano-scaled cementite precipitation formed. On the other hand, a large number of nano-scaled cementite precipitates with the size of 10~100 nm formed in the steel with 0.17%C. Therefore, the precipitation of the nanoscaled cementite precipitates could be realized by the UFC process for the plain carbon steel with 0.17%C but with no request for the addition of microalloying elements. Due to the precipitation strengthening of the nanoscaled cementite, the yield strength of the experimental steels with 0.17%C increased with the lowering the finish temperature of the UFC process gradually and typically reached an increment higher than 110MPa. A further thermo mechanical treatment (TMT) after UFC can increase evidently the dislocation density for cementite nucleation, and it will be a feasible way to realize the uniform precipitation of nano-scaled cementite entirely in the microstructure of the steel, thereby further enhancing the strengthening effect. After hot rolling with the UFC and TMT process, the yield strength of the 0.17%C steel may reach a level greater than 650 MPa, in other words, a net increment larger than 300 MPa may be ascribed to the precipitation strengthening effect of nano-scaled cementite.