Abstract: The parameters which affect the toughness of high grade pipeline steels were studied using electron backscatterea diffraction(EBSD).It is shown that the grains with grain boundary orientation higher than 15° will be helpful to toughness of materials.The effective particle size,the frequency distribution of grain boundary orientation and the ratio of high grain boundary to low grain boundary are important parameters in high grade pipeline steels.That finer grain size,smaller effective particle size and higher frequency of grain boundary higher than 50° would effectively lead to the excellent toughness of high grade pipeline steels.