Surfactant flooding is one of the enhanced oil recovery processes to recover residual oil from oil reservoirs. It is considered that reduction of interfacial tension by surfactants improves mobility of trapped oil in porous media. However, its fundamental mechanism is not clarified. A measurement technique to visualize the two-phase flow of oil and aqueous surfactant solution in porous media was developed by using refractive index matching method. Therefore, the micro-scale dynamics of oil recovery via surfactant flooding was investigated. Mobilization and coalescence of residual oil droplets were observed through surfactant injection.