This study aims to measure the farm specific irrigation water use efficiency (IWUE), through non parametric DEA model; and to evaluate the potential irrigation cost reductions and the main factors causing variations in IWUE among a sample of irrigated farms in Tunisia. Cross sectional data collected from a sample of 75 farms participating in the the WaDImena project in adhour region (northern Tunisia) was used. The results showed that the average level of IWUE across the farm sample was around 61.2%. Farmers would be able to reduce their actual cost by 5% if they adjust irrigation water to its efficient level. This low level of cost reductions is consistent with the existing literature about IWUE in Tunisia. Moreover, education level of farmers, access to credit and agricultural extension service showed a positive relationship with the IWUE in our case study.