Novel diphoshinoamine(PNRNP) ligands have been used in ethylene oligomerization with Cr(III) compounds and cocatalyst. The results indicate that the catalyst system can yield 1-octene in selectivities up to 80%. Another novel diphoshinoamine ligands (((Ar2P)2N)2R)(R=cyclopropylamine) also were synthesized and characterized. Factors which affect the result of ethylene tetramerisation, such as cocatalyst, temperature, pressure and the molar ratio of Al/Cr were examined. Steric effects and electric effects were also considered in this report.