Biodiesel defined as mono-alkyl esters of vegetable oils and animal fats, has had a considerable development and great acceptance as an alternative fuel for diesel engines. Density and viscosity are two important physical properties to affect the utilization of biodiesel as fuel. In this work, mixtures of biodiesel and ultra low sulfur diesel (ULSD) were used to study the variation of density (rho) and kinematic viscosity (eta) as a function of percent volume (V) and temperature (T), experimental measurements were carried out for six biodiesel blends at nine temperatures in the range of 293.15-373.15 K. Both, density and viscosity increases because of the increase in the concentration of biodiesel in the blend, and both of them decrease as temperature increases. One empirical correlation was proposed to estimate the density: rho = alpha.V + beta.T + delta; and three empirical correlations were developed to predict the kinematic viscosity: eta = exp[ln(gamma) + phi.V + omega/T + lambda.V/T(2)], eta = exp[ln(gamma) + omega/T + lambda.V/T(2)] and eta = exp[ln(gamma) + omega/T + lambda.V/T]. The corresponding parameters were optimized by the Levenberg-Marquardt method. The estimated values of density and viscosity are in good agreement with the experimental data because absolute average prediction errors of 0.02% and 2.10% were obtained in the Biodiesel(1) + ULSD(2) system studied in this work. (C) 2011 Elsevier Ltd. All rights reserved.