The experiment was carried out at Bangladesh Rice Research Institute (BRRI), Regional Station, Comilla, Bangladesh to determine the seeding interval of most promising parental lines of hybrid rice during boro 2009-10 season. The seeding interval between A (Cytoplasmic genetic male sterile line) and R (Restorer line) lines were determined by growth duration difference (GDD), leaf number difference (LND) and effective accumulated temperature (EAT) method. Five A lines and 12 R lines were used in this experiment. In 5 A lines, the days to 50% flowering ranged from 120 to 135 days and in 12 R lines, it ranged from 127 to 139 days. The leaf number varied from 15.07 to 18.63 and from 17.05 to 19.89 in the A and R lines, respectively. Among the A line, the EAT ranged from 1069 to 12910C and 1174 to 13510C in R lines. The growth duration differences between A and R lines varied found from 1 to 19 days. Similarly, the leaf number differences between A and R lines were from 0 to 4.82. The seeding differences between A and R lines in terms of EAT were from 13.50 to 282.20C.