The densities of liquid sodium, potasium, and their alloy NaK have been measured, in the temperature range 0-300℃, by the dilatometer method, special attention being paid to temperature control and to the purification of the metals. It is found that the density of the liquid metals Na and K can all be adequately expressed as a linear relation with the temperature, though our values are slightly higher than those given by Hagcn. In the case of the liquid alloy, the three samples (K: 49%, 51%, 74%) give also almost parallel straight lines, but with the exception that as the temperature approaches the melting point of the alloy, the density shows quite systematical deviations, probably indicating some strutural change in the alloy even long before freezing.