Biodiesel is a renewable green fuel obtained through the transformation of animal and vegetable fats, as well as used oil. This biodiesel can replace fossil diesel. The production of biodiesel from recycled oils is comparable in quality to that virgin vegetable oil biodiesel with an added attractive advantage of being lower in price. The current research was therefore to study the production of biodiesel from used frying oil for fried banana in Phnom Penh by reducing the viscosity of this oil by trans-esterification. In this process, there are two main steps. The first step is the esterification by adding a small amount of H2SO4 as a catalyst. The second step is the trans-esterification by using NaOH as a catalyst. The yield of biodiesel obtained from the experimentation was 88% compared to the quantity of used frying oil as a raw material. By transforming the used frying oil to biodiesel, the viscosity decreased from 39.32 mm2/s to 7.93 mm2/s. The other parameters are also improved such as apparence, color, flash point, water content, ash content, acid value and iodine value. In conclusion, the process of trans-esterification can reduce the viscosity of used frying oil that is applicable for biodiesel.