As world oil reserves dwindle, the challenge of producing more oil is increased. The flow behavior in porous media is very complex due to interactions between fluid and packing particles. The essential parameter used to describe the two phase flow in porous media is relative permeability, which is generally dependent on fluid properties like viscosity, wettability, interstitial fluid distributions, saturation history and interfacial tension. In this Thesis, at first, new empirical-mathematical models are developed to predict relative permeability of gas/oil and water/oil systems in different rock and fluid systems using MGGP method. At second step, different experiments are conducted for calculation of gas/oil relative permeability and oil recovery factor to investigate the effect of gas flow rate, interfacial tension, rock type and wettability using sandpack. Another set of experiments are reffered to new topic in petroleum field called Pre-Darcy phenomena. At Pre-Darcy experiments, the effect of permeability, different liquids and gases are investigated. This investigation is presented new results in Pre-Darcy flow field. The results strongly admit the MGGP approach and high accuracy of developed mathematical correlations. In experimental part, in investigation of interfacial tension and rate effect, the results show increasing both oil and gas relative permeability curves by increasing gas flow rate. Rate effect is more pronounced in oil relative permeability (wetting phase). Investigation Pre-Darcy phenomena and consider it in productivity calculation, reduced many errors and causes oil recovery factor improvements. In wettability section, in general, the water wet system, shows minimum oil recovery factor for all gas flow rates. In investigation effect of rock type, it was observed that due to higher porosity and permeability of sandstone, its oil RF was maximum. Also, it was concluded that gas flow rate don’t have any effect on oil RF of sandtone. While for ca