Packed-bed reactors, in which a gas and a liquid down flow concurrently though a randomly packed particles, are widely used in petroleum and petrochemical industrials which mostly operate in trickle flow mode. The interfacial area is the most important parameter since mass, momentum, and energy transports occur at the interface between gas and liquid phases. In order to evaluate the interfacial area in the trickling flow regime, the local film thickness and the shape of the interface are needed. The local film thickness and the shape of the interface are found using the non-dimensional analysis and the simplified geometry of a packed-bed reactor and measuring the film thickness by the electrical conductivity probes. The interfacial area predicted by the presented model is comparable with that of the other empirical correlations. The results show that the interfacial area concentration increases as the flow regime moves from low-interaction to high-interaction flow regime.