A new model has been developed in order to predict pressure drop and gas fraction of industrial fixed bed reactor operated in downward bubbly flow. The mechanistic approach proposed by Attou and Ferschneider (1999) has been followed. In this approach, mass and momentum balances are solved for gas-liquid mixture. To solve the system equation, closure laws are needed for the interfacial force between liquid and solid F-LS and for the gas fraction expression. These closure laws have been determined by measuring both pressure drop and gas fraction with downward air-water bubbly flows through a 40 cm diameter fixed bed of glass beads. The averaged gas fraction is measured by using a gamma scanning system that perform attenuation measurements along 64 chords across a section. Finally, a new bubble flow model is proposed to interpret the results.