Abstract: The microstructure of 1100 MPa grade quenched and tempered(Q& T)high strength steel(Q1100)was studied by metallographic microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM), and the influence of microstructure on the strength of high strength steel was investigated.The results show that, after quenching and tempering at low temperature, Q1100 high strength steel is formed as tempered martensite, and low-temperature tempering retains martensite laths. The average grain size of austenite in high strength tempered steel is 13.6 μm, the average width of the martensite bundle is 1.57 μm, and the average width of the martensite lath is 201 nm.The precipitated phase morphology is mainly short rod shape and spherical shape after low temperature tempering, and the size distribution is 5-50 nm. Due to low temperature tempering, high strength tempered steel avoids the growth of carbide. Q1100 high strength of the quenched and tempered state is mainly due to the multi-layered structure of the steel containing the original austenite grain boundary, martensite package, martensite block and martensite lath, as well as the high density dislocation in the martensitic microstructure and the precipitates at the nanometer level.