Abstract: Problems related with evaluation of surface density, S_v^gb(ext), of "extended grain boundary" have been studied. An equation has been derived to calculate the systematic error introduced by inferring S_v^gb(ext) bed on the surface density of the real portion of grain boundary and the volume fraction of second phase, and three methods for direct measurement of extended grain boundary have for proposed to eliminate the systematic error Some case studies have been conducted and demonstiated that the methods proposed can avoid the inferring error and ensure the accuracy for stereological characterization of the interaction between the grain boundary and the surface of second phase.