The measured properties of biodiesels met with Thai biodiesel (B100) specifications, with the exception of the slight lower methyl ester content and lower viscosity of sunflower-used frying oil biodiesel and coconut-used frying oil biodiesel, respectively. Therefore, these mixtures of fresh vegetable oil and used frying oil can be recommended as a supplementary oil feedstock for biodiesel production in Thailand if engine performance tests provide satisfactory results.