We propose a scalable and energy-efficient microwave-assisted chemical reactor for biodiesel production, which is composed of a partially modified conventional 10-L stainless steel vessel and a microwave coupler to enable an optimized microwave injection of 99% power efficiency. The microwave power applied via a waveguide can be directly injected into the reaction vessel using a coupling rod clamped to a pressured microwave window, giving convenience of scale-up of the reactor volume because a conventional microwave transparent vessel like glass is not need. Microwave-assisted transesterification of triglycerides with potassium hydroxide catalyst achieved an accelerated conversion of 95% in 5min. The precisely measured microwave energy consumption was only 87% of the calculated heat requirement for both the reactant and the vessel. Computer simulation studies indicated that the cause of the energy efficiency for microwave heating was the relatively low temperature of the vessel due to a reverse temperature gradient, in contrast to those done with conventional hot wall heating.