Experimental Study on oil-water two-phase flow patts and pressure loss was conducted on a horizontal steel pipe loop with 26.1mm inner diameter and 30m total length.The working fluids are white oil, diesel oil and tap water.Several instruments, including a new type of liquid-probe are successfully integrated to identify 7 different flow patts.The characteristics of the flow patts and the transition process were observed and depicted in this paper.Investigation revealed that the pressure loss was mainly depended on the flow patts.