•Flow patterns and plug hydrodynamics of liquid–liquid flow are investigated.•A criterion for plug tail cap turns from a curved surface to flat is developed.•The Capillary number has a significant effect on the plug length.•Prediction correlation on plug length is developed in the liquid–liquid system. The flow patterns and the plug hydrodynamics of liquid–liquid two-phase flow in a small circular channel were experimentally investigated. Five distinct flow patterns, including droplet flow, slug flow, throat-annular flow, annular flow and stratified flow were observed in the silicone oil–water system. The flow pattern map was present systematically based on Re, We, water volume fraction and mixture velocity to evaluate competitive relations of internal forces in each flow regime. Three parameters were defined to characterize the plug shape, and the change law of the plug shape was synthesized based on this. A criterion for plug tail cap turns from a curved surface to flat was developed. Capillary number in the liquid–liquid system has been proved to affect the plug length prediction significantly. A new plug length prediction correlation was summarized through the two linear fits of the plug length under different flow ratios. The water-isooctanol and water-dioctyl phthalate systems were selected to correct the plug length prediction correlation. The predicting results agree well with the literature data, and the maximum error is about 19.7%.