Cr(III) catalysts supported by asymmetic N,N-diphospholylamine ligands bearing a phenoxaphosphine group are highly active for ethylene tri-/tetramerization with considerable selectivity upon activation with MMAO-3A. Asymmetric PNP ligand with a N-cyclohexyl group achieved the highest activity of 282.2kg/(gCr/h) with a high total selectivity of 83.2% toward valuable 1-hexene (28.7%) and 1-octene (54.5%) at 35 bar under 80°C. Display omitted We have developed novel Cr(III) catalysts supported by asymmetic N,N-diphospholylamine ligands bearing a phenoxaphosphine group. Upon activation with MMAO-3A, the Cr(III) catalysts supported by the PNP ligands are highly active for ethylene tri-/tetramerization with considerable selectivity. The ligand substitution and oligomerization conditions are found to be essential to achieve high activity and controllable selectivity. The catalytic system with asymmetric diphospholylamine ligands exhibited higher activity than that supported by symmetric ligands. Asymmetric diphospholylamine ligand with a N-cyclohexyl group achieved the highest activity of 282.2kg/(gCr/h) with a high total selectivity of 83.2% toward valuable 1-hexene (28.7%) and 1-octent (54.5%) at 35bar under 80°C.