This experimental research was to test a performance of flat-plate solar collectors using biodiesel as a working solution with a solar thermal system. The purposes were to promote a biodiesel use as an alternative fuel for higher efficient heat energy and as an added renewable energy for a production process and to provide a guideline in education and for development in the future. The experiment was to test a performance of flat sheet solar collectors using ASHRAE STANDARD 93-2003 and to test a thermal potential of 12 litres by volume of biodiesel in a 2 m2 flat- plate solar collector used as heat source of the system. Thus biodiesel was used as a working solution with a flow rate of 2.4 litre per minute and the test duration was between 09:00 am. and 03:00 pm. The result revealed the performance values of the flat-plate solar collector used in this experiment as FR( ) = 0.958 and FRUL = 15.02 W/m2K. The temperature of biodiesel in the solar thermal system increased from 33๐C to approximately 80-90๐C varied with solar radiation values. However; the mentioned degrees could be efficiently applied in biodiesel production process as a main heat source or a supplemental heat source depending on appropriate variables.