Agri-food waste is an exciting way to use as a polymer filler. It is an ecological option for cutting down the material cost, especially relatively expensive biodegradable plastics. In this paper, lignocellulosic residues of chokeberry pomace (CP) and beetroot pulp (BP) are grafted with lactic acid oligomers (OLAs) in direct condensation of lactic acid (LAc) into the lignocellulosic backbone. Biocomposites with modified and unmodified fillers were prepared using poly (lactic acid) (PLA) matrix. Additionally, the influence of an epoxy-based chain extender (Joncryl® ADR 4468 (ADR)) on the properties of the biocomposites mentioned above was checked. SEM was used for the characterisation of modified and unmodified fillers. The success of the grafting reaction was confirmed by FTIR and 1HNMR analyses. DSC, DMA and TGA was applied for the characterisation of thermal properties of produced biocomposites. The influence of chemical modification and introduction of ADR to the biocomposite on water absorption, rheological and mechanical properties were tested. Moreover, the reaction between ADR and the lignocellulosic backbone of the CP filler was studied by FTIR analysis. Results showed, that modification of the filler using OLAs has a significant influence on reducing water absorption by 48.3 and 56.5 % for CP and BP biocomposites, respectively. The tensile strength increased after the modification and introduction of ADR. The chemical treatment of the filler causes an increase in the tensile strength by about 29 and 8 % compared to unmodified CP and BP fillers, respectively. 0.75 % of ADR in CP/PLA biocomposite allowed for an increase in tensile strength by 51 %. Rheological study showed that presence of ADR compensates the consequences of hydrolysis and segment exchange of the PLA chains caused by water and OLAs, respectively. Results are promising for the production of ecological biocomposites with satisfying mechanical, thermal and rheological properties compared to biocomposites with unmodified lignocellulosic filler.