In machine scheduling problems, not all machines are always available. They may be interrupted periodically for reasons such as planned maintenance, lunch and rest breaks. However, this situation is often ignored in the literature or these time periods are considered as fixed. In this study, flow shop scheduling problem with unavailable time periods is discussed. In the considered problem, unavailable time periods are not fixed, they can be pulled forward to prevent the idle times. A MIP model and a genetic algorithm have been developed for the considered problem. The performance of the proposed solution methods is demonstrated using randomly generated test problems. With the developed genetic algorithm, GAMS results are improved by an average of 26,82%.