The objective of this study was to determine the coefficient of irrigameter device (KI) and to assess its performance in the irrigation of sunflower. The experiment was conducted in the Jaíba Irrigated Perimeter, Jaíba, MG, located at 15° 36' S and 43° 42' W, with an altitude of 460 m. For the evaluation of the performance of the irrigameter, the water level in the evaporimeter was kept at 1, 2, 3, 4, 5 and 6 cm high, and the evapotranspiration estimated by the irrigameter was compared to the reference evapotranspiration, calculated by the standard method Penman-Monteith FAO 56 (Allen et al., 1998). It was concluded that the values of KI obtained were 0.42, 0.69, 0.85, 1.07, 1.34 and 1.61 for the water levels in the evaporimeter corresponding to 1, 2, 3, 4, 5 and 6 cm, respectively. There was no significant difference between the yield of sunflower crop irrigated during day or night time; higher average values then reported by the literature were found. The irrigameter device was found to be satisfactory for irrigation control. To estimate the reference evapotranspiration in similar regions with similar climatic conditions, it is recommended to use the unit with evaporimeter adjusted to the level 3.7 cm.