The rapid growth of railway based public transportation in metropolitan city leads to the massive development of its supporting facilities such as underground tunnel. The minimum exposure towards ambient air in the underground tunnel encourages further studies about the conflagration smoke behavior which affects visibility change. In the existing situation, the smoke extraction system is supported by forced ventilation, which has been qualitatively known to be able to extract smoke during the evacuation time. The studies about the natural ventilation towards visibility change in fire conditions at the underground tunnel become interesting to be explored due to the possibility of mechanical smoke extraction system failure during evacuation stages. The fire laboratory test is carried out at a 1:16,25 scale tunnel model and numerical simulation using Fire Dynamics Simulation version 6 with 1:16,25 scale. The Froude Number is kept to be constant for both of the methods by controlling the heat release rate to ensure the ratio between fluid inertia force towards its gravitational weight is similar. In this case, the full-scaled smoke behavior can be generally predicted by the laboratory scale results. The different characteristic in visibility declination among the methods as the result indicates an inaccuracy of the simulation method to predict the visibility declination characteristic by the time. It is only generally able to predict whether the visibility decline or is constant by the time. The final conditional conclusion can be highlighted from this research is that results from the simulation method must be validated by the experimental results in predicting visibility declination characteristic in underground tunnel fire scenarios.