The tensile properties of the cellulose-polymer composites are presented in Table 1. As seen, the addition of 5 wt% of cellulose to matrix material determines the increase of the Young’s modulus E that overcomes the corresponding standard deviations. At lower content of cellulose no significant effect is observed. It is detected that with increasing cellulose content the modulus Ebecomes up to 12 % and 30 % higher for PLA/CELL and PE/CELL, respectively, compared to matrix polymers indicating the reinforcing action of the filler. These results are in accordance with the rheological data on storage modulus G/. Such behaviour is expected because it is well known that the modulus of a filled system depends on the properties of the two components, the filler and the matrix [18, 19]. Thus, the E of the filler (cellulose), being higher than the E of the matrix materials, causes the increase in Eof composites. Increasing modulus of elasticity of composites compared with matrix polymer can also be associated with the restrictions of macromolecules mobility and deformability imposed by the presence of cellulose particles, and this is in agreement with obtained rheological data. Since the Young’s modulus is a measure of the material stiffness, we can say that with increasing cellulose content the materials become stiffer. At the same time the PLA and their composites having much higher Eare much stiffer than LDPE and its composites.