The properties of biocomposites based on ethylene-octene copolymers (EOCs) were studied. The mechanical, rheological, and physical properties of the biocomposites were investigated with attention to the filler type, filler content, and grade of EOC. Three grades of EOC with different viscosities were used as the polymer matrices. Two lignocellulosic raw materials were selected as fillers: birch wood flour (WF) and oil flax straw (FS). The effect of the filler variation range (30%-70%) on the biocomposites was studied. According to microphotographs, the filler particles were quite uniformly distributed in the polymer matrix. The WF particles had ellipsoid shapes, whereas the FS particles were characterized by cylindrical shapes. The addition of fillers decreased the elongations at the copolymer's break and tensile strengths, but increased the elasticity moduli. The fillers' hard particles increased the stiffness of the polymer chains in the biocomposites, resulting in declines in elongation at break. The biocomposites with WF had higher tensile strengths and elongations at break than those with FS because the latter were characterized by lower density values than the former. Melt viscosity analysis showed that filler content increased with melt viscosity. The FS biocomposites had higher melt viscosities than the WF biocomposites. POLYM. ENG. SCI., 57:756-763, 2017. (c) 2017 Society of Plastics Engineers