The main goal of this paper is to present the methodology of construction of the Integral Indicator for the Croatian Thermal Power Plants and the Combined Heat and Power Plants. The Integral Indicator is intended to compare the Power Plants according to a certain criterion. The criterion of the ecological impact is chosen. The following features of the power plants are used: generated electricity and heat; consumed coal and liquid fuel; sulphur content in fuel; emitted CO 2, SO 2, NO x, and particles. The linear model is used to construct the Integral Indicator. The model parameters are defined by the Principal Component Analysis. The constructed Integral Indicator is compared with several others, such as Pareto-optimal slicing indicator and Metric indicator. The Integral Indicator keeps as much information about the waste measures of the power plants as possible; it is simple and robust. ► Ecological impact of the Croatian Power Plants is estimated. ► Several methods for integral indicator construction were developed. ► Expert estimations of the integral indictors were compared with the model results. ► The new method of ecological impact assessment was proposed.