The present investigation is carried out to study the swelling and thermal behaviours of alkali treated cellulosic fibre (sisal and jute) with fly ash dispersed unsaturated polyester composites. These composites are fabricated by compression moulding with a filler loading of 5, 15, 25, 35, and 45 wt. %. The electronic thermal insulation tester, differential scanning calorimeter, and thermo-gravimetric analyzer are used for thermal analyses of the synthesized composites. It was observed that the thermal stability and degradation temperature of the composites improved significantly with addition of the filler within the unsaturated polyester resin.The swelling behaviours are determined by immersing the synthesized samples in the different pH content of water. In case of swelling behaviours, the changes in water absorption are quite acceptable as compared to the treatment time and the atmospheric condition. The fabricated composites displayed optimum results for filler content with 35 wt. % followed by saturation in properties with the dispersion above the same.