The possible decomposition of metastable austenite during the partitioning process in the highend quenching and partitioning(Q&P)steels is somewhat neglected by most researchers.The effects of primary martensite and alloying elements including manganese,cobalt and aluminum on the isothermal decomposition of austenite during typical Q&P process were studied by dilatometry.The transformation kinetics was studied systematically and resulting microstructures were discussed in details.The results suggested that the primary martensite decreased the incubation period of isothermal decomposition by accelerating the nucleation process owing to dislocations especially on phase and grain boundaries.This effect can be eliminated by a flash heating which recovered dislocations.Co addition significantly promoted the bainite transformation during partitioning while Al and Mn suppressed the isothermal bainite transformation.The bainite transformation played an important role in carbon distribution during partitioning,and hence the amount and stability of austenite upon final quenching.The bainite transformation during partitioning is an important factor in optimizing the microstructure in Q&P steels. The possible decomposition of metastable austenite during the partitioning process in the highend quenching and partitioning (Q & P) steels is somewhat neglected by most researchers. The effects of primary martensite and alloying elements including manganese, cobalt and aluminum on the isothermal decomposition of austenite during typical Q & P process of studied by dilatometry.The resulting kinetics was discussed systematically and results microstructures were discussed in details.The results suggested that the primary martensite decreased the incubation period of isothermal decomposition by accelerating the nucleation process owing to dislocations especially on phase and grain boundaries.This effect can be eliminated by a flash heating which recovered dislocations. Co addition significantly promoted the bainite transformation during partitioning while Al and Mn suppressed the isothermal bainite transformation. The bainite transformation played an important role in carbon distribution dur ing partitioning, and consequently the amount and stability of austenite upon final quenching. bainite transformation during partitioning is an important factor in optimizing the microstructure in Q & P steels.