At present, no fieldmeasured values are available in China and abroad for the modified altitude coefficient for areas with more than 2400m altitude. Regarding Zhegushan tunnel, which is an extralong highway tunnel with high altitude, the authors fieldmeasured the smog altitude coefficient for areas with 400m to 4000m altitude from 2003 to 2005 and studied on the reasonable value of smog altitude coefficient in the operation ventilation design of highway tunnels. The study can offer reliable basis and useful reference for the operation ventilation design of Zhegushan tunnel and some other similar highaltitude highway tunnels.