Drilling and blasting is widely used in underground excavation projects, where the amount of damage to the surrounding rock mass is crucially important, due to its impact on the safety of working environment and operational costs,. The causes of overbreak in the Alborz Tunnel of Iran are evaluted. In this regard, ten rounds of presplitting and 11 rounds of smooth blasting methods were carried out to determine the dominancy of ground condition over the blasting pattern characteristics. Further study was undertaken to identify the most important parameters of ground condition affecting the overbreak area. These parameters include; joint condition, spacing, orientation, RQD and type of rock mass. As the characteristics of the blasting pattern have very little effect on the amount of overbreak, the smooth blasting technique was chosen for the future operations where the current ground condition is going to be dealt with for about 500 meters of length, based on the data acquired from the Alborz Exploratory Tunnel. Results of this investigation helped to solve disputes between contractors and clients over the issue of permissible overbreak.