Investigation of nano liquid-metal fluid (consists of liquid metal Ga and nanoparticles copper) as heat transfer medium in circular tube is performed for the first time. The numerical simulations of heat transfer enhancement of nano liquid-metal fluid in a circular tube subject to a constant wall heat flux are carried out, and the heat transfer performance is evaluated. The two-phase mixture model is used to simulate the flow of nanoparticles–liquid mixture for Reynolds number (Re) from 250 to 1000 and nanoparticle volume fraction (α_{p}) from 0 to 0.1. The results show that the average heat transfer coefficient of nano liquid-metal fluid Ga–Cu is 23.8 times of that of nanofluid water–Cu at Re = 500 and α_{p} = 0.04, and the average wall shear stress of Ga–Cu is 0.0154 Pa, whereas for water–Cu, it is 0.0259 Pa. As Re increases from 250 to 1000, the average heat transfer coefficient of water–Cu is improved by 40%, whereas for Ga–Cu, it is 45.4%. Based on the results in the paper, the nano liquid-metal fluid can be considered as an excellent heat transfer medium of forced convection in circular tube.