(1) The relations between the dimensional changes of carbon steel bars due to heating and cooling and the A1∼A3 or Acm transformations were investigated. (2) If the rate of heating and cooling is either extremely small or large as in air cooling, the percentages of dimensional changes of low carbon steel bar are small, as compared with the case of intermediate cooling rate. (3) It is considered that the banded structure of carbon steel bar is a factor which has influence on these dimensional changes.