Process quality control utilizes inspection of a product while it is being produced through testing periodic samples of products. After this, if product quality characteristics have been changed, the process is stopped, and a search is made for an assignable cause. The objective of a process-capability study is to assess the ability of a process to meet product specifications. During a quality improvement initiative, capability-estimates are obtained at start and at end of study to reflect the occurred improvement. Several capability-estimates show how a process is capable of meeting its specification limits. Essentially, capability-estimates reflect the non conformance rate of a process in a single number form. This involves calculating some ratio of process specification limits. In this study, the production of a fluorescent lamp consisting of a glass tube is considered. Tube quality is affected by the production line conditions. So, the maintenance of the production line is an important stage to increase tube quality. In this paper glass tube quality is studied. Minitab is a statistical software was used to analyze obtained data. The results showed an obvious improvement of part quality after maintenance stage. All measurements were done in Toshiba factory for fluorescent lamp manufacturing.