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Biodiesel fuel is a non-fossil fuel and an important renewable fuel. It can be obtained from vegetable oil or animal fats, and has recently been suggested as an alternative fuel for immediate utilization as a surrogate for petroleum-based fuels. The combustion of biodiesel fuel or blended with diesel fuel in compression ignition (CI) engines is general results in lower smoke, PM, CO and HC emissions compared to standard diesel fuel, while NOx emission is greater in different extent. Flame temperature of biodiesel fuel combustion plays a very important role on the formation of NOx emission, which is described by Zeldovich mechanism. Thus, studying temperature profile during biodiesel combustion can provide useful insights to the formation and destruction of NOx. The main objective of this work is to investigate the effect of blending biodiesel fuel with diesel fuel on open air flames temperature. The experiments were carried out by thermocouple measurements of local flame temperatures of wick generated open air flames. The relationships between blend proportions and flame temperature of different layers, number and distribution of high temperature points, relative and mean flame temperature, adiabatic flame temperature, rate of fuel combustion were obtained. In general, it was found that along with the proportions of biodiesel fuel in the blends increases, temperature of outer flame, numbers of high temperature points and their corresponding area, relative and mean flame temperature, adiabatic flame temperature are all increased, while adiabatic flame temperature is decreased. The main reasons are due to higher oxygen content and lower calorific value of biodiesel fuel.