Several flow regimes exist in a gas-liquid cocurrent downflow in a packed bed (trickle bed). These are given descriptive names such as trickle, pulsing, spray and bubble. The hydrodynamic and transport properties of trickle bed are dependent on these flow regimes. The hydrodynamic modeling of gas-liquid cocurrent downflow through a packed bed based on pore scale is presented. The pore scale hydrodynamic is related to the macroscopic scale flow parameters. The two-phase flow at pore level channel is modeled similar to annular two-phase flow in tubes. Using the film instability due to shear by the gas at the pore level channel, the transition criterion for the trickle flow to pulse flow regime is obtained. The model predictions agree with the published data. A model for transition from partial wetting to complete wetting in trickling flow was presented. The model predictions are in agreement with previous models. The model indicates better wetting of the smaller size packing for the same liquid mass flux.