Abstract: Effects of heat input and interpass temperature on cryogenic toughness of four sub-zones in heat affected zones (HAZ) were investigated for 9% Ni steel welding.Optical microscope and X-ray diffractometer were used to analyze types and quantitative distribution of microstructures in the sub-zones.Results showed that cryogenic toughness of inter-critical HAZ (ICHAZ)was not sensitive to the change of heat input and interpass temperature.The main technique parameter influencing cryogenic toughness of coarsegrained HAZ (CGHAZ)was interpass temperature, whereas the main technique parameter influencing cryogenic toughness of super-critically reheated coarse-grained HAZ (SRCGHAZ)and inter-critically reheated coarse-grained HAZ (IRCGHAZ)was the heat input. Higher heat input and longer remaining time at high temperature resulted in much coarser grain size and much lower toughness.Higher interpass temperature and stronger self-tempering effect of martensite resulted in higher cryogenic toughness.A match of low heat input and high interpass temperature is recommended to achieve high cryogenic toughness.