The tiploma thesis "The EU policy to fight public sector corruption in the member and in the candidate states: an instrument to reduce corruption or a cause of a paradoxical membership effect?" deals with the conception, the development and the current state of the EU anticorruption policy in the member and in the candidate states. It provides an introduction into the issue of corruption and its control, with a particular emphasis on the corruption in the post-communist countries of Central and Eastern Europe. The thesis explains the interest of the EU in this phenomenon and provides an overview of the EU instruments to fight corruption in the member and in the candidate states. At the same time, it compares the effectiveness of the two dimensions of the EU anticorruption policy. Exemplified by the case of the post-communist states that entered the EU in 2004, the thesis analyzes whether the EU accession is connected with weakening of obligations that arise from this policy. Due to possible negative consequences of such a paradoxical membership effect, the thesis searches for possibilities how to reduce it and how to prevent it in the future. The thesis also provides for a brief case study on corruption and anticorruption in the Czech Republic from the application for the EU membership until now.