Abstract The fracture mechanics analysis of surface cracks is important for the integrity evaluation of flawed structural components. The objective of this paper is to numerically investigate the interaction effect of twin surface cracks in plate and cylindrical geometries. First the usefulness of the line-spring model is verified by analyzing a single surface crack in a plate, and then the model is extended to twin surface crack in plate and cylindrical geometries. For the case of a finite plate under uniaxial loading, the effect of crack spacing on the stress intensity factor is negligible. However, for the case of a cylinder under moderate internal pressure, a significant increase in stress intensity factor is observed at the deepest point of the surface crack.