ZnO has been reported to be the optimum heterogeneous catalyst for mitigating harsh supercritical operating conditions for biodiesel production because of its high activity and negligible leaching in supercritical methanol. However, more data on transesterification using ZnO in supercritical methanol is still needed for industrial applications. In this study, biodiesel was produced by the transesterification of soybean oil with a ZnO catalyst in supercritical methanol. The kinetic parameters were determined at the initial stage of the reaction to examine the reaction rate, and the activity of the recycled catalyst was measured to determine its reuse performance. The effect of the free fatty acid content was also investigated to validate the feasibility of applying waste oils to the biodiesel feedstock.