The increasing energy demand has led to the utilization of part of Brazil's energy water potential, and in this context the exploitation of the Rio Claro basin, one of the main tributaries of the Paranaíba River basin, located in the southern region of Goiás state framed, and is the target of this study the reservoir of Barra dos Coqueiros hydroelectric power plant, located in the middle portion of this basin. The enrichment of water in nutrients is called eutrophication, and its measurement is through the calculation of the Trophic State Index (IET). The objective of this research is to evaluate and classify the IET of the Barra dos Coqueiros Hydroelectric Power Plant reservoir through the proposed method. Carlson (1977), and modified by Toledo (1984) and Lamparelli (2010) in two distinct periods, dry (July 2012) and rainy (January 2013). The analyzed variables are: phosphorus, chlorophyll a and Secchi transparency depth, which were applied in the equations proposed by each method. The IET of the chlorophyll variable a, predominated two classes in all proposals in the two analyzed periods, which was oligotrophic and mesotrophic. The total IET in Carlson's (1977) and Toledo's (1983) proposal had little variation and oligotrophic and mesotrophic predominance in the dry and eutrophic and mesotrophic periods in the rainy season. Already in Lamparelli (2004), all classes were diagnosed in both periods. In all variables analyzed and applied to the proposals, the IET was higher in the rainy season than in the dry period.