1/20 scale model test and full scale field test were carried out to study the critical velocity for fire ventilation in horizontal tunnels. Combined with the test results, smallscale test results by Jae and fullscale test and simulation by Hu, a semiempirical model was developed to predict the longitudinal critical velocity for tunnel fire ventilation. Compared with the predictions of the proposed model and other models using air as fire source, it was found that the predictions by Wu and Barker was underestimates.