The world is looking for changes in its energetic matrix through the use of biofuels to reduce the dependence on oil derivatives. Currently, the number of studies concerning mixtures of triglycerides from animals or plants with diesel fuel has risen due to efforts to reduce the consumption of diesel oil. This paper studies the behavior of the density of plant oils mixed with diesel fuel in relation to temperature, which is essential to guarantee engine efficiency. The density is related to the mass and to the volume being a critical property to the assessment of the subministered energy to the engine by the injection system, which behaves like a pump of positive displacement. Samples of the diesel fuel mixture were made with plant triglycerides in proportion and range of ∼5–20%. The densities of the samples were measured in a density meter with temperature varying between 18 and 25°C, as well as at 15°C, according to ASTM standard. Afterwards, the uncertainties of measurement were calculated, and the density change with respect to temperature is presented and shown in density variation curves. An analysis of the density variation curves shows that.