Abstract: In order to learn the contribution of different reinforcement modes,after hot rolling with mill and tempering,the microstructure,dislocation density,second phase morphology and size of low carbon bainitic steel were investigated by using SEM,TEM and positron annihilation technology.The yield strength was quantitatively calculated.The mechanical properties of the tested steel were determined by using multi-function material testing machine.The results show that the dislocation density was about 2.65 × 1014 m-2.Dislocation strengthening which was the main mode contributed 327 MPa to the yield strength,which accounted for 41.3% of the yield strength.A large number of fine spherical and near-spherical (Ti,Nb)(C,N) second phase particles were observed and the size of most particles was less than 10 nm,which contributed 172 MPa to the yield strength and accounted for 21.7%.Solution strengthening and interstitial atom strengthening contributed 129 MPa and 94 MPa to the yield strength,which accounted for 16.3% and 11.9% respectively.The strength value calculated by the strengthening mechanism model was very close to the measured value.