During the past decades, there have been an acceleration of customers’ needs of customized products. This have pressured many companies in offering customized products in order to reach customer satisfaction and keep growing and expanding their market share. Nowadays the competitive pressure between companies with the same product segment is increasing and the market requirements are changing rapidly. To handle this competition, many manufacturing companies are focusing on mass customized production. However, mass customization has been a topic of interest for many researchers. The focus has been in studying and understanding the complexity and the constrains that a company encounter when dealing with mass customized production in mostly the automotive industry. One of the constrains that have been less highlighted by researchers were the effects that mass customized production has on the output variation of production lines. Therefore, the purpose of this thesis is to investigate how to minimize output variations in mass customized production within assembly lines. To do that the researchers focused on studying the output variation in whitegoods industry instead of the automotive industry that captures most of the existing research. The study includes two main research questions. The first question was to identify the factors that affect the output variation in mass customized production. While the second question was to identify applicable solutions that can be used to stabilize the output. To ensure the reliability and validity of the research findings, the researchers used multiple case study combined with literature reviews. To answer the research questions several data collection techniques were used in the multiple case study conducted on two lines. Those data collection techniques were participating observations, document reviews, a semi-structured interview and many conversations with the affected persons. The findings from the case study for research question one showed consistency with the theories described in the theoretical background. This was evident because similar problems when it comes to output variations were defined in the theories. One problem was about the simplification of assembly line balancing problem that was evident in both case studies. This was an effect of the missing product family classification which created a high output variation because of the use of only one line balancing for all product variants. Another problem identified included functionality problems of machines because of performing wrong assembly or the use of the wrong component. There was as well the problem of missing or delayed components which was an effect of the lack of structure in the material picking and the deliveries performed for both lines. A combination of those factors created the experienced output variation on both lines. To deal with those problems and minimize the output variation, a solution approach was defined in the second research question. This approach was created together with the case company with the help of the theoretical background and it presents several steps to follow when attacking and resolving similar problems as presented above. The most important conclusion of this thesis is that, when dealing with output variation in mass customized production where people are performing the assembly, it is essential to provide the right conditions for them to ensure that they have the right knowledge base to perform the requested assembly.