The changing global world and accordingly, the increasing consumer demands and needs indicate that companies should adopt continuous improvement as a principle in order to stand sustainable on their sector. Lean Six Sigma (LSS) provides effective and sustainable solutions to meet world standards with the tools and techniques it uses. In this study, which focuses on process improvement, it is aimed to optimize the costly flexible polyurethane foam production process conditions and find out which factors are effective on the hardness of flexible polyurethane foam produced according to slabstock method by reducing variability on this process. At the end of the study, it was understood that the significant factors in the foam process were TDI index and polyol. This study improves the process approximately 15 Newton. This means better use of resources, quality and happy customers.