The present paper deals with polymerization of ethylene by chromium acetylacetonate Cr(acac)(3) - activated by alkylaluminium, e.g. Al(C2H5)(3) or Al(i-C5H9)(3), or methylaluminoxane (MAO). The influence of polymerization parameters on catalyst performance, such as Al/Cr mole ratio, temperature, aging time, type of cocatalyst, was investigated. High activities were obtained only when MAO was the cocatalyst. The polymers were characterized by size exclusion chromatography (SEC) and differential scanning calorimetry (DSC) analyses.