Detailed experiments of fracture toughness in which SENB specimens of five different thicknesses were included were carried out to investigate the size effect in the ductile to brittle transition temperature region. It is found that the fracture toughness of the upper shelf increases with the thickness of the specimens with the similar geometry. While the fracture toughness of the lower shelf decreases with the thickness in the range of 4mm to 12mm and then drops up from 12mm to 16mm with the appearance of shear lips which present the shearing fracture under the plane stress state. The tearing modulus dJ/da which determines the resistance to stable crack growth increases with the increment of thickness and the reduction of the temperature. The results of the stress triaxiality increasing with the reduction of the thickness explain well the experimental results.