The aim of this work was to investigate the effect of reactive agents on the rheological behaviors, morphology and mechanical properties of two biodegradable polyesters; poly (lactic acid) (PLA) and poly (butylene succinate) (PBS) reinforced with eucalyptus fiber. Two types of reactive agents, chain extender (Joncryl®) and peroxide (Perkadox® 14), were selected. Biocomposites consisting of a constant fiber loading of 10 wt% with and without reactive agents were prepared by one-step melt-mixing methods via an internal mixer with the rotational speed of 60 rpm and the temperature of 150 °C and 190 °C for PBS and PLA, respectively. The biocomposite samples were then compression-molded for tensile testing. Rheological and morphological properties of the biocomposites were also investigated by means of dynamical mechanical analysis (DMA) and scanning electron micrograph (SEM). The results revealed the presence of Joncryl® in PLA biocomposites caused the increase of mixing torque, complex viscosity, storage modulus and mechanical properties when compared to the addition of peroxide. The increase of complex viscosity of PLA biocomposites added Joncryl® was due to the increase in the population of the higher molecular weights as confirmed by GPC analysis. In the case of PBS biocomposites, it showed a dramatic increase in the complex viscosity and storage modulus with decreasing crossover frequencies when adding peroxide rather than adding Joncryl®. The SEM micrographs demonstrated that the compatibility between the polymer matrix and fiber was improved when adding a small amount of Joncryl® and peroxide for both polymers, except for 0.5 phr peroxide loading, leading to an improvement of their modulus and tensile strength.