Strength-deformation characteristics of low-density polyethylene filled with microcrystalline cellulose “Thermocell” as a function of the TC content (up to 0.7 parts by weight) are studied. Characteristics such as elastic modulus, relative elongation at break, ultimate strength, and work of failure are determined. Water sorption and change in the size and strength-deformation characteristics of composite specimens during exposure to boiling water (560 min) are also studied. It is shown that with greater filler content it is possible to increase the strength-deformation characteristics of LDPE, such as elastic modulus and tensile strength. The growth of the ultimate strength is associated with the formation of a specific filler framework with increasing filler content. It is found that the main factors which cause a decrease in the elastic modulus and softening of the composite are failure of the filler framework as well as formation of stresses and voids during water sorption by the composite. It is demonstrated that the steady reproducibility of the composition, attainable high filling degrees, and ecological safety make Thermocell a promising filler for polyethylene.