In the face of continuously rising energy prices, depleting fossil fuel resources and environmental problems caused by them, interest in alternative energy sources is increasing day by day. Vegetable oils are one of the important alternative energy sources. Furthermore, these oils can be used as fuel in diesel engines. One of the biggest problems of using vegetable oils as fuels is their high viscosity value. Transesterification is a good way to decrease their viscosity and density. Early prediction of viscosity of biodiesel and its blends with diesel fuel saves time and effort, and also provides correct blending rates for diesel engines. In this study, through neutralization process, neutralized waste cooking oil (NWCO) was obtained. Then, biodiesel was produced from NWCO via transesterification method using sodium hydroxide (NaOH) and methyl alcohol. The resulting biodiesel was mixed with diesel fuel by means of volumetric ratios of 5, 20 and 50%. Using a Koehler brand K23377 viscosity measurement device, kinematic viscosities of B100, B50, B20, B5, diesel and NWCO were determined to be between the ranges of 303.15-373.15 K for each 10-K interval. Additionally, kinematic viscosities of B100, B50, B20, B5, diesel and NWCO were estimated by means of Arrhenius blending equation. Kinematic viscosity changes depending on the temperature, absolute errors and percentage error rates were determined by using Andrade equations with 2 and 3 constants via MATLAB R2008a package program. Kinematic viscosities of 50, B20 and B5 fuels, obtained by the Arrhenius equation with an experimental value, in the temperature range of 303.15-373.15 K, were compared and maximum error rates (10.18, 6.086 and 7.809%) were determined, respectively. When 2-constant Andrade equation was used, the maximum error rates for B50, B20 and B5 fuels were determined to be 8.398, 4.744, and 4.879%, respectively, and R-2 value was determined to be >0.9913. When 3-constant Andrade equation was used, the maximum error rates for B50, B20 and B5 fuels were determined to be 5.753, 1.632 and 1.022%, respectively, and R-2 value was determined to be >0.9969.