Ethylene/dodecene-1 and ethylene/butene-1/dodecene-1 copolymers were prepared and their polymerization reaction in heptane slurry was studied. The Ziegler-Natta catalyst was aluminium-activated TiCl//3, and the cocatalyst was triethylaluminium. The dodecene-1 contents in the ethylene/dodecene-1 copolymers were less than 3 wt %. The dodecene-1 contents in terpolymers of ethylene, butene-1, and dodecene-1 ranged up to 10 wt %. The dependence of basic properties on composition of terpolymers was investigated.