Abstract: The paper researches the influence of liquid viscosity on gas-liquid two-phase flow in horizontal loop of 52 m-long stainless steel pipe with an inner diameter of 25.7 mm. The five flow patterns observed in the test include air mass flow, stratified flow, stratified wavy flow, slug flow and wave flow. In the case that calculated speed for the gas is same with that of the liquid, along with the increase of liquid viscosity, the long bubbles in air mass flow become longer, the height of liquid film of stratified flow and slug flow decreases, the slug frequency increases, the average liquid level and amplitude of flow wave increases and the speed and frequency of the wave are reduced. The gas-liquid two-phase flow pattern map under different liquid viscosities is plotted. There is no stratified wavy flow found when the liquid viscosity is not less than 20 mPa·s, and with the increase in liquid viscosity, the area of stratified flow is gradually decreased, the transition boundary from air mass flow into slug flow is drifted to the direction of small gas speed, the boundary for generating wave flow is drifted to the direction of liquid speed and high gas speed, easily forming a slug flow. Comparison verification with T-D model is performed, which is not suitable for flow pattern calculation under the experimental condition when the liquid viscosity is relatively large.