The purpose of this paper is to explore the issue of the criteria of project success and the complexity of the subject in the context of environmental and nature conservation projects financed by European Union. The article presents various definitions of project success The article deals with definition and evaluation process of the project success as well as specific conditions of EU project management. Thematic evolution and trends in defining project success are presented through systematic review of literature on project management. The first part of the article focus on reviewing different approaches to the subject of criteria of project success, which is the crucial part of the proces. It is impossible to determine critical success factors (CSF) without deciding on the criteria of the project success. Project success definition is an important and complex project management issue. The success of the projects was considered for the last 50 years in a various ways and by different project management scholars. There is a consensus about the importance of this aspect for the project management practice. However, the project management in the context of EU-funded projects is still subject of further research, as this issue was not yet properly analyzed. The article presents the specific of the environmental and nature conservation EU-funded project judgement proces. The procedures and the scope of the formal and substantive assesment which is the part of selection procedure were described. The paper presents also how formal and substantive assesment criteria correspond with success criteria definitions created so far by the researchers. Moreover, the article analyses how existing asssesment criteria, precisely defined in EU programmes documentation and procedures, can be treated as EU project success criteria. The article presents also the most important challenges and issues in determining the set of success criteria based on assesment criteria set for regional operational programmes and Operational Programme Infrastructure and Environment for 2014–2020 programming period.