The theme of this master thesis is "Six Sigma" methodology and its application on reducing the number of corrugated cardboard packaging complaints. This paper will explain the basic concepts of "Six Sigma" methodology and its implementation according to the DMAIC method. There will be described some of the tools used for the implementation of the methodology and their use through five stages for improving an existing process (Define, Measure, Analyze, Improve and Control). The application of the methodology for solving problem of corrugated cardboard packaging complaints will be presented. The first part will present the corrugated cardboard, raw materials for its making, corrugated cardboard types and types of waves inside it. The second part will present the technique of flexographic printing and machinery used for the processing of corrugated cardboard. The third part will present the "Six Sigma" methodology, its history, levels of certification and the role of the "Six Sigma" projects. Special emphasis will be placed on the DMAIC method, explaining all its stages and some of the tools used. The DMADV method, implemented when designing a new process, and its stages will also be mentioned (Define, Measure, Analyze,, Design, Verify). The experimental part will include work on an actual project named „Reducing the number of product complaints” which aims to reduce the number of corrugated cardboard packaging complaints. The project will be implemented through the DMAIC stages with the tools used during the implementation of the "Six Sigma" methodology.