An experimental investigation was conducted on the effect of injection timings and pressure on flame temperature of biodiesel. To estimate flame temperatures accurately during the combustion processes, an optical HSDI diesel engine composed of an endoscope and a high-speed CCD camera was used. In addition, the AVL-ThermoVision software allowed measurement of discrete flame temperatures from images taken during the combustion processes. NOx and soot concentration, which are directly related to the flame temperature, were also measured using exhaust emission analyzers, in order to study the correlation between emissions characteristics and the temperature under various injection timings. The results of this study show the characteristics of flame temperature values and distribution for biodiesel and ULSD fuel owing to the difference of their physical properties and injection strategies. Additionally, the study of high pressure injection was performed under the same conditions. When the injection pressure increased, more homogeneous combustion was observed. (C) 2013 Elsevier B.V. All rights reserved.