This study aimed at optimizing the mixture of palm kernel shell (PKS) and selected additives (Al₂O₃, MgO, and CaO), on the yield of ash after combustion using D-optimal under the Mixture Methodology of the Design Expert (6.0.8). The samples were oven-dried at 105°C, then placed in furnace at 575°C and ashed to a constant weight based on the experimental design. Al₂O₃ (5.0%), MgO (5.0%), CaO (5.0%), PKS (85.0%) and Particle Size 1.0 mm gave the highest yield (1.39%) of ash while the combination of Al₂O₃ (2.5%), MgO (0.0%), CaO (5.0%), PKS (92.5%) and Particle Size ( 5.50 mm ) gave the lowest yield (0.31%) of ash after combustion. The regression coefficient (R²) of the model equation obtained for the process is 0.2985, while the predicted R² is 0.1673, which shows a reasonable agreement with the adjusted R² (0.2902). The results of optimization indicate that mixed ratio of additive such as Al₂O₃, MgO, and CaO with PKS and particle size influenced the yield of ash after combustion and the essential additive are Al₂O₃ and CaO. This study has therefore indicated the efficacy of using additive mixed with PKS to achieve lowest ash yield and therefore reduce negative impacts of increased ash deposition on the efficiency and operation of a combusting unit.