Ventilation when driving roadways is one of the most important considerations in coal underground mines. Ventilation efficiency depends on fan performance and ventilation ducting system. In recent years in Vietnam, ducts have often been produced domestically. However, parameters of the duct as duct leakage, duct resistance for designing auxiliary ventilation system are referenced from abroad handbook. This lead to inaccurate design results and needs to adjust efficiency of auxiliary ventilation during driving roadways. Determination of duct resistance; air leakage in ducting system have been undertaken. The research result has been used to optimize the auxiliary ventilation system.