A hydrodynamic model for cocurrent gas-liquid downward flow through a packed-bed reactor operating in trickle flow regime is developed. The model involves area-averaged mass and momentum balance equations of each fluid as well as an expression for the capillary pressure gradient deduced from a momentum balance analysis at the gas-liquid interface. The liquid-solid and gas-liquid interaction forces are formulated on the basis of the Kozeny-Carman equation by taking into account the presence of liquid films and the gas-liquid slip motion. The linear stability analysis of the solution of the proposed model around an equilibrium steady state is applied for obtaining the trickling-to-pulsing transition curve. The theoretical predictions of the trickling-to-pulsing transition are compared favourably with a set of experimental data corresponding to a wide range of operating conditions, fluids properties and packing characteristics.