The crystallinity of ethylene-butene copolymers prepared by copolymerization of ethylene and butene in the presence of a new highly active catalyst was studied by means of DSC, WAXD and DMA. The results show that the melting temperature, the crystallinity and the crystallite size decreased with increasing the content of butene in the copolymers. The copolymers have a high degree of branching, the butene segments are mainly in the amorphous regions of the copolymers, while the polyethylene sequence forms crystal phase acting as crosslinking bondage between the molecules at room temperature. The ethylene-butene copolymers have a low modulus, a low stress and a high strain analogous to the stress-strain behavior of non-cross thermoplastic elastomer. The crystallinity of ethylene-butene copolymers prepared by copolymerization of ethylene and butene in the presence of a new highly active catalyst was studied by means of DSC, WAXD and DMA. The results show that the melting temperature, the crystallinity and the crystallite size decreased with increasing the content of butene in the copolymers. The copolymers have a high degree of branching, the butene segments are mainly in the amorphous regions of the copolymers, while the polyethylene sequence forms crystal phase acting as crosslinking bondage between the molecules at room temperature. ethylene-butene copolymers have a low modulus, a low stress and a high strain analogous to the stress-strain behavior of non-cross thermoplastic elastomer.