Based on the three existing forms of liquid in a trickle bed, as proposed by Charpentier et al. (1968a,b), a straight channel model is developed with a wall constituted from the packing. Within the channel liquid and gas phases are assumed to flow concurrently downward at constant velocities, just as in an empty pipe. Since the channel wall is assumed to be formed from the packing, the capillary influence could be taken into account. Capillary and frictional forces are considered as the two principal factor which determine the stability of the liquid film covering a pellet. Evaluating the balance of these two factors gives rise to a new model which is considered the necessary condition for pulsing flow inception corresponding to the original work by Ng (1986).