In investment projects of public importance, as well as in all other projects, it is important to make a cost-benefit analysis of the investment. Cost-benefit analysis as a method of assessment of public projects is strongly encouraged by the European Union. The aim of the European Union is to achieve economic growth and create new jobs, and in order to achieve these goals it is necessary to choose projects for co-financing that will bring the best results and contribute to the objectives of balanced development of all European Union member states. In projects of public importance, which apply to European Union (EU) tenders, this method is a fundamental part of the feasibility study and a mandatory part of the application documents, because the EU’s goal is to fund projects with the greatest benefit to society. Cost-benefit analysis is mandatory for large projects, while for smaller projects instructions are given in various guides for implementation of a simplified cost-benefit analysis. The aim of this paper was to analyse the theory of cost-benefit analysis, to explore the application of the methodology of the Guide to Cost-benefit Analysis of Investment Projects and the Guide to Feasibility Studies for Irrigation Projects, which are the fundamental documents for analysis of projects of public importance, particularly irrigation projects, and find out to what extent the Guide to Feasibility Studies for Irrigation Projects gives precise instructions in the procedures for conducting feasibility analysis of project proposals. The paper provides an overview of the literature, gives a definition of investment projects and financial analysis, describes the discounted cash flow method and explains the importance of financial analysis in the investment decision-making process. The paper also describes the method of calculating social benefits through conversion factors and shadow prices, provides the steps in conducting economic analysis and gives an example of cost-benefit analysis on the example of an irrigation project with analysis of the Guide to Feasibility Studies for Irrigation Projects. At the end of the paper, recommendations are made for amendments to the Guide to Feasibility Studies for Irrigation Project with the aim of methodological harmonization of analysts’ actions in the development of the analysed projects. The amendments to the Guide should result in quality analyses with the correct methodology applied, and selection of projects that bring the most benefit to society.