Nanofluids which are suspension of nanoparticles in conventional heat transfer fluids attracted researchers while they show higher thermal conductivity and specific heat capacity. The important parameters have influence on thermal-fluid properties of nanofluids include the volume fraction of the nanoparticles, temperature, density of fluid base and nanoparticles, nanoparticles size, nanolayer, thermal conductivity of base fluid and particles, and pH. Nanolayer which is an approved interfacial layer between particles and base fluid involved in some of modelling for effective thermal conductivity and effective viscosity of nanofluids. Therefore, this layer must effect on other properties of nanofluids such as density. In this study investigation into the density of nanofluids has done experimentally. The nanofluids were investigated for density measurements consist of SiO2-Water, MgO-Glycerol, CuO-Glycerol and SiOx-Ethylene Glycol /Water for range of 1 to 6% volume fraction as well as temperature range of 10 to 40oC. The results show that mixture model for density of nanofluids (density of nanofluid = density of base fluid multiply by volume fraction of base fluid + density of nanoparticles multiply by volume fraction of nanoparticles) which is generally cited in literature has higher value than experimental data. For higher volume fraction of nanoparticles, the gap between the experimental results and the mixture model gets more. This is due to the nanolayer which also shows nanolayer density can be between void and the base fluid density. Therefore, based on the experimental data a new model for density of nanofluids developed which includes nanolayer. It was also found that the amount of the void in the nanolayer is more sensitive to nanoparticle size and not to base fluids or nanoparticles material.