Effects of nitrogen on yeast cell morphology, settleability and performance of wastewater treatment were investigated in treating oil-containing wastewater by yeast-SBR system. The results show that: nitrogen supply affects directly yeast biomass, settleability, pH and treatment efficiency of system; the absence of nitrogen induces the transformation of certain yeast cells from single cell to hypha morphology. Based on an overall consideration of efficiency and stability of yeast-SBR system, the optimum BOD/N ratio of influent is 20/1. The optimum nitrogen supply can improve wastewater treatment efficiency of systems with different degrees of nitrogen absence and make yeast morphology become the predominant morphology for slightly mycelial system over a short time, but for severe mycelial system, the hypha morphology still keeps dominant.