Indonesia has an abundance of natural biomass resources, particularly from oil palm agriculture, which is underutilized. Empty fruit bunches (EFB) and fronds (FRD) are two examples of insufficiently utilized biomass, both biomasses are blended in equal proportion and coded as EFFR. Even though cocombustion is frequently examined using a single variety of biomass, barely any article thoroughly investigates the effect of combining multiple types of biomasses. For assessing the co-combustion effect, five blends of EFFR were employed, with combination 5% (EFFR5), 10% (EFFR10), 15% (EFFR15), 20% (EFFR20), and 25% (EFFR25), with sub-bituminous coal. All samples were examined in duplicate for characteristics analysis (proximate, ultimate, and calorific value) then thermogravimetric analysis employed TG–DTA. As the results, the EFFR20 combination exhibited the optimum sample as combustion evaluation result, with highest index value at ignition index (Di), burnout index (Db), combustion index (S), reactivity (R), flammibility index (C), while lowest value at index of intensity (Hf), with 4.78 × 10–8%/sec3, 20.88 × 10–12%/sec3·oC, 11.44 × 10–11%/sec2·oC3, 4.77 × 10–3 mg/sec, 3.58 × 10–5%/sec·oC2, and 2.74%·oC2/sec, respectively. The findings of this study reveal that, up to a 25% EFFR mixture, the combustion properties are excellent. However, in terms of optimum combustion performance, a composition of 20% EFFR is thought to be most appropriate for these requirements.