‘Palbangmi’ is a super high-yield rice cultivar bred by interspecies cross between Tongil type and Africa rice. But, ‘Palbangmi’ has a problem in grain maturation which results in rice yield decreases. Because ‘Palbangmi’ is Tongil and Africa rice interspecies-crossed cultivar, it would be predicted to require higher temperature for grain maturation than japonica rice. Therefore, ‘Palbangmi’ would be required to determine Late Marginal Heading Date (LMHD) for normal maturation. In this study, experiment of transplanting date was carried out to determine LMHD of ‘Palbangmi’ in Suwon and Iksan for 3 years. When daily mean temperature was below 23oC, grain filling ratio (%) increased about 8% for each 1oC. On the other hands, grain filling ratio (%) didn't increase when the daily mean temperature was above 23oC. Therefore, it is determined that 23oC is adequate for minimum threshold temperature for grain maturation. The LMHD of ‘Palbangmi’ was determined from calculated heading dates for each year which could satisfy threshold temperature for 40 days after heading during last 15 years. As a results, LMHD was earlier than August 1 in 16 regions where it is inadequate for ‘Palbangmi’. Continually, LMHD was August 1 ~ August 5 in 13 regions, August 6 ~ August 10 in 14 regions and August 11 ~ August 15 in 21 regions. Finally, LMHD was later than August 15 in 14 regions. Considering that heading date of ‘Palbangmi’ is August 13 as mid maturing ecotype, it is suggested that earlier transplanting required for normal maturing in almost areas except south plain area and some areas of central plain area.