Abstract: An observation was made of the microstructure in an ultrahigh strength steel after continuous cooling and low temperature isothermal transformation. The results show that there exists an aging stage in martensite and bainitc during continuous cooling or low temperature isothermal transformation,Its structural feature is tweed pattern. The orientation relationship between the transition carbide and matrix does not conform to the well established Jack orientation relationship. but a new one. Transition carbides may emerge directly from the tweed structure.