The density of liquid NiCoAlMo quaternary alloy with a fixed molar ratio of Ni:Co:Al (xNi:xCo:xAl≈73:12:15) which is close to the average value of the commercial Ni-based superalloys TMS75, INCO713, CM247LC and CMSX-4, and molybdenum concentration change from 0% to 10% (mass fraction) was measured by a modified sessile drop method. It is found that the density of the liquid NiCoAlMo quaternary alloy decreases with increasing of temperature, but increases with the increase of molybdenum concentration in the alloys. The molar volume of liquid NiCoAlMo quaternary alloy increases with the increase of temperature and molybdenum concentration, and shows a negative deviation from the ideal linear mixing. The liquid density of NiCoAlMo quaternary alloy calculated from the partial molar volumes of nickel, cobalt, aluminum and molybdenum in the corresponding Ni-bases binary alloy are in good agreement with the experimental results; it means that within the error tolerance range the density of liquid Ni-based multi-component alloys can be predicted from the partial volumes of elements in Ni-based binary alloys in liquid state