Grain boundaries and triple junctions are the most favoured hydrogen trapping sites in the microstructure as they have the lowest trapping activation energy. To study the effect of these trapping sites, different microstructures with different surface ratio of grain boundaries were generated. In order to do this, hydrogen diffusion in both grains and grain boundaries was modelled using Cellular Automaton technique combined with the finite difference method. The simulation was implemented in two-dimensions (2D) in both grains and grain boundaries to highlight the significance of grain boundaries in hydrogen diffusion. The developed model was validated by comparing the simulation results with the experimental outcomes of hydrogen permeability measurements in X70 pipeline steel having different grain sizes. The results revealed that grain size (grain boundary fraction) has a two-fold influence on the diffusion rate, that is, in both very fine and very coarse microstructures diffusion rates are slower than for an optimum intermediate grain size.