A process for preparing crystalline ethylene (co)polymers comprising (co)polymerizing ethylene in the presence of carried out in the presence catalyst system comprising (a) a solid catalyst component comprising Ti, Mg, halogen, ORI groups, where RI is a C1-C12 hydrocarbon group optionally containing heteroatoms, having ORI/Ti molar ratio of at least 0.5, an amount of titanium, with respect to the total weight of said solid catalyst component, higher than 4% by weight, and showing a specific pattern of the SS-NMR; and (b) an aluminum alkyl compound as a cocatalyst. The process allows to obtain in good yields ethylene polymers with narrow MWD.