An important factor in determining water demand and optimum water management at different growth stages of sugar beet SEED-bearing plants is crop coefficient (Kc). The crop coefficient (the ratio of sugar beet SEED-bearing plant evapotranspiration (ETc) to grass evapotranspiration (ET0)) was calculated at different growth stages. The study was carried out by 6 lysimeters located in research station of Karaj Soil and Water Research Institute, Meshkinabad, Karaj in 2006. Two lysimeters where grass had been cultivated were used for determining ET0. To determine ETc of sugar beet SEED-bearing plants, vernalized roots of sugar beet (200-250 g) were planted in four lysimeters. About 2000 and 1000 m2 were planted by the same crop around the lysimeters of sugar beet SEED-bearing plants and grass, respectively. The moisture contents of all lysimeters were measured during the growth period and also after harvest at the depth of 0-60 cm. To compensate soil moisture deficit, the irrigation was carried out after depletion of 40% of available moisture. In addition, precipitation, daily temperature, relative humidity, wind speed, sunny hours and evaporation from Class A pan was measured in a meteorological station located around the grass lysimeters during the growing period. The measured crop factors included root number, total biomass weight, shoot dry weight, root dry weight, flowering stem weight, SEED total weight, SEED size, standard SEED weight, 1000-SEED weight, vigor, germination rate and SEED viability. ET0 was calculated by evaporation pan and applying pan coefficient (Kp) of Karaj and also by Penman & Mantis method and was compared with the data of grass lysimeter. ETc, ET0 as well as Kc were estimated at different growth stages. The results showed that there was great conformity between two methods of ET0 calculation, i.e. by software CROPWAT and measurement. But at the beginning of the study, the ET0 calculated by grass lysimeters was lower than that calculated by meteorological data. However, over the time, the latter was affected by climatic parameters of temperature and sunny hours and increased with a stronger gradient than the former. This period was from the 50th day on. The results indicated that crop evapotranspiration decreased at the initial stage because of undeveloped shoots and at final stage and at the end of growth period because of shoot yellowing and loss of green area and the need for the decrease in SEED moisture during maturation. At the second stage, the crop absorbed greater amount of water and as a result, its evapotranspiration increased because of the fast growth of sugar beet SEED-bearing plants and the proliferation of aerial organs. At the third stage, since on one hand the leaf area of sugar beet SEED-bearing plants was at its peak and on the other hand, the other organs were green, the evapotranspiration of the crop was in its peak. At initial stages of the growth, ET0 was greater than ETc of sugar beet SEED-bearing plants; however, the gradient of these two curves became almost constant over the time and ETc started to sharply decrease from the beginning of the final stage (i.e. about 90th day). The Kc of sugar beet SEED-bearing plants at initial, development, middle and final stages was 0.29, 0.89, 1.12 and 0.66, respectively. Sugar beet SEED-bearing plants consumed 35.13, 141.93, 273.59 and 65.49 mm water at initial, development, middle and final stages, respectively. Their peak water consumption was at middle stage with a rate of 7.82 mm/day.