Fatty acid methyl ester (FAME) was produced from waste cooking palm oil (WCPO) using KOH catalyst supported on palm kernel shells ash (PKSA). Response surface methodology (RSM) based on Box Behnken design (BBD) was used to design the experiment and analyse three parameters: Catalyst amount, KOH loading and methanol to oil molar ratio. The result found that the catalyst activity depended on basicity of the catalysts. The statistical model predicted that the highest yield of 96.73% was achieved with an optimised parameter of 15 wt.% catalyst amount, 20 wt.% of KOH loading and 11.39:1 methanol to oil ratio at the temperature of 65^oC. Experiments performed at the predicted condition yield of 92.74% suggest that the RSM is an accurate method for biodiesel production. The Gas Chromatography Mass Spectrometry (GC-MS) was used to identify the chemical composition of biodiesel. The catalyst reusability test verified that the synthesised catalyst could be reused up to 3 times. The results demonstrated that waste cooking palm oil can be completely transformed into methyl ester. This study indicates that KOH supported on palm kernel shell ash is an effective catalyst for the conversion of waste cooking palm oil to biodiesel.