Background & Objectives: Different methods can be used for ventilation of tunnel during construction. Each method has advantages, limitations, and disadvantages that should be considered in selection of methods. Inefficient ventilation can lowered production rate and provide undesired work conditions for employees. This issue is critical in gaseous tunnels and may imperil the workers lives and the whole project. This paper presents a series of guidelines for selection of ventilation system, optimal implementation, and aspects of ventilation system design in gaseous tunnels based on common ventilation standards. Methods: Common ventilation systems in tunnels during construction were compared for selection of Alborz tunnel ventilation system. Air-supply ventilation system was selected as the best design. Whereas air-supply ventilation system may be applied in different forms, six distinct designs were compared for Alborz tunnel. In order to determine tunnel ventilation capacity, air volume required was calculated based on different criteria.Air volume required for diesel equipment was considered as the air requirements of tunnel ventilation. Considering advantages, disadvantages, and limitations of the small and large diameter channel applications, ventilation system calculations were conducted for the diameters 100, 140, and 180 cm to select optimum size. Results: The results from comparison of different ventilation variants showed that using two parallel jet fans on tunnel surface in a joint canal (140 cm) is the most appropriate design for tunnels with these characteristics. Conclusion: Poor ventilation is a critical factor in deep underground and gaseous tunnel drilling and improper ventilation may substantially affect the work environment, life, and production rate.