The aim of the work was to study the biodegradation process of biocomposites prepared from renewable resources and the ecotoxicological assessment of their biodegradation products. Biocomposites from modified starch reinforced with various cellulose fibers were prepared by the extrusion process. Biodegradation studies were carried out according to the ISO respirometic method. Ecotoxicity of biodegradation products was assessed by the luminescent bacteria test. It was found that biodegradation of biocomposites was above 60% within 24 days according to the results of respirometric test. Increase in the amount of natural fiber reinforcement, as well as smaller fiber size increased the biodegradability of biocomposites. On the basis of the preliminary results of the ecotoxicological test using luminescent bacteria it seems that the biodegradation products of the biocomposites studied are ecologically safe.