Room temperature tensile tests were carried on the hot-rolled state ultra-low carbon and low alloy cabainite and martensite steels which were get by different finishing temperatures and different cooling methods.We used the Scanning Electron Microscopy (SEM),Electron Backscattered Diffraction (EBSD) and X-Ray Diffractometer (XRD) to identify the metallographic structure and analyse the precipitated phase.The inherent mechanism of high strength of ultra-low carbon and low alloy bainite and martensite steels was discussed,and the analysis indicated that the reinforcement of ultra-low carbon and low alloy bainite and martensite steels was mainly produced by the superposition of the dislocation strengthening,solution strengthening and grain refinement strengthening. Room temperature tensile tests were carried on the hot-rolled state ultra-low carbon and low alloy cabainite and martensite steels which were obtained by different finishing temperatures and different cooling methods. We used the Scanning Electron Microscopy (SEM), Electron Backscattered Diffraction (EBSD ) and X-Ray Diffractometer (XRD) to identify the metallographic structure and analyze the precipitated phase. the inherent mechanism of high strength of ultra-low carbon and low alloy bainite and martensite steels was discussed, and the analysis indicated that the reinforcement of ultra -low carbon and low alloy bainite and martensite steels was mainly produced by the superposition of the dislocation strengthening, solution strengthening and grain refinement strengthening.