The degree of matrix-filler interaction plays an important role in determining the mechanical properties of biocomposite materials. To produce highly compatible chitosan biocomposites, 1,2,4,5-benzene tetra carboxylic chitosan (BTC) was used as a filler to reinforce a chitosan matrix (Cs), because the similar backbone structure of Cs and BTC filler is expected to increase the compatibility of the biocomposite. The mechanical properties of these biocomposites were investigated under tension and tearing mode. Introduction of BTC filler (at 0, 2, 4, 6, 8, and 10 wt%) provided dramatic increments in sigma(R) and E values, but the observed values of epsilon(R) and K correlated negatively to similar levels of BTC filler. Morphological results demonstrate that the breaking surfaces of biocomposites form rugged deformation lines, resulting in high sigma(K) values of the biocomposites. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 111-126, 2011