The viscoelastic relaxations of four copolymers of ethylene and I-octadecene, with different comonomer contents, synthesized with a metallocene catalyst, have been analysed and compared to those of the corresponding polyethylene homopolymer. Similar to other ethylene copolymers, the temperature for alpha relaxation decreases with the increase in the comonomer content, attributed to the reduction of the crystal size. In fact, a linear relation has been found between the temperature location of alpha relaxation and the crystal thickness estimated from the SAXS long spacing. However, the position of beta relaxation experiences very little dependence with the comonomer content and it overlaps to alpha relaxation for the higher comonomer contents. As to gamma relaxation, there is also a clear effect of the comonomer content on the breadth and on the location of this relaxation.