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Sedimentary rock is commonly well recognized as one of hydrocarbon reservoir rock. Meanwhile, igneous rock is typically found in geothermal reservoirs. Both rock types have different formation mechanism, which leds to different structural properties. Pore space in the sedimentary rock is formed in between the sedimented grains after undergone a diagenesis (sedimentation, compaction, cementation). Igneous rock however, has pore space mainly in the form of fractures and cracks as a result of high overburden pressure which leds to the various scale of deformation in the rock. In this study, both type of rock were represented by computer models. Pore structure characteristics of both rock type were analyzed with varying porosity. The pore space of the igneous rock is modeled by varying the aperture of the crack, which was originated from a real igneous rock (model C). The sedimentary rock is modeled using the random penetrable sphere model (model G). Pore structure characterization by means of digital image analysis shows that pore structure separation (equals to crack aperture in model C, and pore diameter in model G), specific surface area (surface density), and fractal dimension has clear and distinct pattern with respect to change in porosity. Absolute permeability was then calculated by means of simulating fluid flow through the models using Lattice Boltzmann Method which implemented in Palabos (parallel lattice Boltzmann solver). The absolute permeability of model C shows greater effect due to change of porosity compared to model G. Three dimensional qualitative analysis was also conducted from the simulation results, which portrays the pattern of fluid flowpath through the pore space.