Crop coefficient of sesame is necessary for the water requirement estimation in irrigation water planning and management. This study has been initiated to determine the crop coefficient (K c ) of sesame in a semi-arid climate. The relationships between K c and ET p /E p (pan evaporation) and leaf area index (LAI), growing degree-day (GDD) and days after sowing (DAS), were also investigated. The seasonal ET p for sesame in the study area with a 5 month growth period was 910m. The mid-season and late-season K c values for sesame were 1.08 and 0.64, respectively. These values are somewhat lower and higher than those for other oil seed crops. The K c value for the initial stage was close to that obtained by the procedure proposed by Allen et al. [Allen, R.G., Smith, M., Pereira, L.S., Pruitt, W.O., 1997. Proposed revision to the FAO procedure for estimating evapotranspiration. In: The Second Iranian Congress on Soil and Water Issues, 15-17 February 1997. Tehran, I.R. of Iran, pp. 1-18]. The ratio of ET p /E p varied between 0.49-1.0 from the beginning to the middle of the growing season which is a sign of mild local advection in the region. The maximum ratios of ET p /ET 0 and ET p /E p occurred at a LAI of 3.0. Furthermore, third-order polynomials were presented to predict the K c values from days after sowing (DAS), percent days after sowing (%DAS) and growing degree-day (GDD).