The rice stem borer, Chilo suppressalis Walker, is the most important rice pest in north parts of Iran and survives the winter as diapausing mature larvae in rice stubble and weeds. To determine the strategy of hibernation and changes of cold hardiness, overwintering larvae were collected from a paddy field at Rice Research Institute in Rasht from October 2004 to March 2005. Because of hard winter in 2004 and snowfall about 2m on February no sampling was done on this month. The supercooling points (SCPs) of whole body of the larvae collected on October was -18.8∞C but it significantly increased to -12.4∞C on November and did not change until snowfall on February. Thereafter, SCPs was raised to -8.2∞C on March. There was more than 90% survival in the larvae collected from October to March, 24h after exposure to -10 and -15∞C but the survival was increased from 30% on October to 80% on November and the following months, 24h after exposure at –20∞C. Because of the increase of supercooling point in winter and endurance of temperaure below this point it is suggested that this pest to be a freezing tolerant insect in Iran and entered diapause from late November in the year of study.