Abstract: The effects of planting time on endogenous hormones contents and oxidase activities in tetraploid Robinia pseudoacacia softwood cuttings were carried out to elucidate the rooting mechanism and determine the optimum planting time for rooting at experimental nursery of College of Forestry, Northwest Agriculture and Forestry University in 2010The results showed that IAA contents of softwood cuttings planted in July and August were higher than those in May and June. High IAA content was an important factor inhibiting rooting. ABA contents were not found to inhibit rooting, but to assist rooting. The higher ZR contents were got with the earlier planting time. Furthermore, IAAO activities of softwood cuttings planted in August were obviously higher than that in May, June and July. PPO activities were closely related to rooting. PPO activities were higher in early planting time than those in later. POD activities and the lignification of softwood cuttings were increased with planting time. Thus, POD activities could serve as an indicator of lignification degree.From the results obtained,it can be recommended that the optimum planting time for tetraploid R.pseudoacacia softwood cuttings is ranging from middle of May to July.