In tunnel excavation, contour hole blasting is generally accomplished by smooth blasting to minimize the blast-induced stress and fracturing of rock beyond the excavation line. In order to successfully conduct smooth blasting, several factors have to be reasonanbly determined such as types of explosives, spacing between contour holes and ratio of spacing to burden. Among these factors, choosing ans appropriate spacing between contour holes according to its surrounding rock mass condition is essential in tunnel excavation. The objective of this work is to numerically investigate the optimal spacing between contour holes in five different rock mass class environments using Particle Flow Code. The extent of damage zone ans propagation of cracks were monitored. Crack information such as failure mode, location, density, and counts was tracked and analyzed. As a result, the optimal spacing between the contour holes was determined to be around 0.6 m in the cases of rock mass classes I and II, and 0.7 m in the cases of classed III, IV, and V.