Natural clay comprises a blend of diverse minerals exhibiting distinct structures and characteristics. Montmorillonite exhibits strong acidity and stabilising effects on active species due to its unique layered structure, thus making it a suitable material for use as a heterogeneous catalyst. Therefore, this research aims to examine the effect of separation and thermal modification process of Tanah Datar clay minerals, as well as their catalytic activity in the conversion of waste frying oil (WFO) into biodiesel. The catalysts were evaluated using X-ray Diffraction (XRD), X-ray Fluorescence (XRF), and Laser Particle Size Analyser (LPSA). The uncalcined clay primarily consists of montmorillonite, along with kaolinite, hematite, and quartz. Upon calcination, montmorillonite transforms into meta montmorillonite. The particle size of montmorillonite increases after separation and calcination. The catalysts were employed in a lab-scale biodiesel production via transesterification reaction, employing a mole ratio of oil and methanol of 1:6, catalyst amount of 3% (w/w), stirring speed of 500 rpm, and a temperature of 70°C for 3 hr. Both uncalcined and calcined montmorillonite catalysts can enhance biodiesel yield by up to 80.11%. This observation demonstrates the considerable potential of the implemented treatment as a catalyst in biodiesel production.