Disclosed is a new titanium based catalyst system for (co)-polymerizing ethylene or ethylene with alpha-olefin having 3 to 10 carbons. The catalyst is obtained by contacting a magnesium halide support with an aluminum compound and with a titanium halide compound and then treating the resulting solid with a magnesium-amide complex. The catalyst system is suitable for producing ethylene polymer and co-polymer with narrow molecular weight distribution as well as improved branching compositional distribution.