An important method of the modification of the properties of polyethylene and polypropylene is the copolymerisation with alpha-olefins. The alpha-olefin behaves as a short branch in the structure, and the rheological and thermal properties of copolymers change significantly. A new method is suggested to evaluate the rheological and thermal properties of copolymers on the basis of homopolymer properties, by introducing a short branch degree parameter. The zero shear viscosity, plateau modulus, fractional free volume, reptation tube diameter, glass transition temperature and thermodynamical melting point were calculated.