Abstract: The Charpy V-notch impact tests were performed under a series of temperature for samples with different size cut from Q245R steel plate to demonstrate the effect of size. The results show that the impact absorption energy of samples with different size was neither convertible nor corresponding to the cross-sectional area of samples. Furthermore, the data of ductile brittle transition temperature for samples with different size also could not be compared directly. With the decrease of sample size, the ductile brittle transition temperature would move to lower temperature. Samples with small size may lead to material toughness overestimating at low temperature. Therefore, it is suggested to adjust the test temperature for samples with small size.