The late marginal heading date (LMHD) for normal maturation has been assessed to infer the range of late transplanting date and applicability of a new cropping system. A minimum threshold temperature for grain filling can be used to determine the late marginal heading date that would allow normal maturation. In this study, a statistical procedure was developed to determine the threshold temperature for 40 days of the grain-filling period. The threshold temperature was chosen to be the minimum temperature at which grain-filling ratio and weight are minimally affected. To identify the threshold temperature, grain-filling ratio and weight, which were observed at two sites for 2 years in Korea, were compared with a range of cumulative temperature during 40 days in the late season. As a result, cumulative temperature of $800^{circ}C$, which is equivalent to, on average, $20^{circ}C$ during 40 days of grain-filling period, was determined to be the threshold temperature for normal grain filling in the late season. The LMHD for normal maturation was also determined from cumulative probability density function of dates from which the threshold temperature of $800^{circ}C$ was met during the last 15 years as well as 33 years. Our results suggested that it would be better to use the cumulative density function of those dates derived from the last 15 years rather than 33 years because current climate conditions could be better represented by the density function derived from recent years.