This paper analyzed the interaction between two parallel vertical cracks in a transversely isotropic half space by using the boundary element method (DBEM). The crack surface is perpendicular to the free surface and isotropic plane subjected to normal and tangential uniform distributed load respectively. The stress intensity factor (SIF) values of the crack are calculated from the crack opening displacements. And the interaction effect factor of SIF values is employed to quantitatively describe the interaction between two cracks. The variations of interaction effect factor are investigated with the distances between cracks, the side ratios and free surface. Results show that the existence of the free surface exerts more obvious influence on the SIF values of the crack close to free surface in half space than those in infinite medium. But the free surface has almost no influence on the interaction between two cracks.