In order to study the effect of restricted irrigation systems and different amounts of potassium fertilizer on water use efficiency and yield of rapeseed (Brassica napus L.), an experiment was conducted in an arid area in Khuzestan, Iran in 2013. The main plots consisted of three irrigation methods: FI (full irrigation), alternate furrow irrigation (AFI) and fixed furrow irrigation (FFI). Each subplot received three rates of K fertiliser application: 0, 150 or 300 kg ha-1. The results showed that the plots receiving full irrigation produced significantly higher grain yield, 1000-kernel weight and grain number per pod than both alternate treatments. However, the highest WUE was obtained with alternate furrow irrigation and 300kg K ha-1 and the lowest was found in the FI treatment and 0 kg K ha-1. Potassium application increased RWC in alternate furrow irrigation and fixed furrow irrigation more than the FI treatment. Maximum oil content was observed in those treatments where full irrigation was applied while minimum oil content was produced in FFI irrigated treatments. Potassium fertilizer also increased grain oil by 15% over the control. Deficit irrigation reduced oleic acid and erucic acid; however oleic acid and linoleic acid increased with increasing potassium.