Biodiesel as a pure, non-toxic, biodegradable and renewable alternative for fossil diesel fuel has attracted much attention in recent decade. Thus, demands for researches in this field are growing up every day. In order to simplify the mentioned research a new method was introduced to determine progress and end point in transesterification of sunflower oil to biodiesel (methyl esters) by the use of physical property variation during reaction. This method can be replaced by expensive and time-consuming, quantitative analysis stage. In the present work first transesterification of sunflower oil at 65 degrees C with MeOH to oil molar ratio of 6:1 and 1 wt.% of KOH as catalyst under vigorous mixing at different durations was carried out to determine how conversion and physical properties change. It was concluded that this reaction proceeded over 90% in 5 min and most of the changes occurred in this short period. In the second step, to verify physical properties variation in all ranges, six blends of produced biodiesel and sunflower oil were prepared in different wt.%, as incomplete reaction mixture. Finally appropriate functions were fitted on the extracted data and were evaluated. Refractive index and specific gravity were selected as good physical properties to predict reaction progress. (C) 2011 Elsevier B.V. All rights reserved.