Deep cryogenic treatment (DCT) is a supplement to conventional heat treatment, which usually involves cooling the material to liquid nitrogen temperature around -196 ℃ for a given soaking time and then heating back to the room temperature. As claimed in many pioneering researchers, DCT can evidently improve the hardness and wear resistance of high-carbon high-alloy steel and has been widely used to die steels, cutting tools, carburizing steels and barrels. The improvement of mechanical properties by DCT can be attributed to the transformation from retained austenite to martensite, the fine dispersion of nanoscale carbide precipitate and the removal of residual stresses. However, the nanoscale carbide precipitate is still lack of evidence and the interpretation of carbon segregation behavior during DCT is still unconvincing. In this work, the high-carbon high-alloy steel SDC99 is first austenized at 1030 ℃ for 30 min and then immersed in liquid nitrogen for 8 h and finally tempered at 210 ℃ for 2 h. The spatial distributions of carbon atom and alloy element concentration in quenched, DCT treated and tempered samples are analyzed by three dimensional atom probe (3DAP), respectively. In addition, the axial ratio and carbon content of martensite are studied using XRD and the carbide morphology before and after DCT are also observed in situ by SEM. The results indicate that after quenching from 1030 ℃ to room temperature, the volume fraction of retained austenite in SDC99 is about 21.1%. The retained austenite is soft and unstable which can easily transfer to martensite at lower temperatures. Carbon atoms will segregate slightly due to self-tempering. However, other alloy atoms do not segregated with carbon atoms. After quenching from 1030 ℃ to room temperature and then cooling in nitrogen for 8 h, the volume fraction of retained austenite in SDC99 will decrease to 7.4%. Carbon atoms will segregate along the twin boundary of martensite and form a segregation area with a thickness about 5~10 nm.There is no carbide precipitate after DCT. Furthermore, carbon atoms segregate again during heating up back to room temperature from -196 ℃. After tempering at 210 ℃ for 2 h, the volume fraction of retained austenite is almost 5.4%. Both carbon and alloy atoms will segregate during tempering at 210 ℃. With the increase of tempering time, the carbon segregation will aggravate and result in a C-rich phase or form the M₂₃C₆ carbide combined with other alloy element. This is one of the main reasons increasing the wear resistance of tool steels.