Although Internet booking system has reached its peak in the recent years many previous researches say there is always a growing interest in the role and nature of design in HCI principles. Small difference can create a large impact in the business of Internet booking system. The future of any online booking system will be heavily depended on how easy and how friendly the system is designed. In our thesis, we are going to suggest what aspects of usability principles are important in making the user interface of current flight booking systems to reach a high degree of usability. For the above purpose we will find usability flaws in the current user interface design of flight booking systems and highlight the potential factors or aspects that make user interface more acceptable to users. To reach this purpose we will use a case study and analyze using an explanation building technique. The empirical data was collected through satisfaction questionnaire which was based on nine “preset tasks” which are designed depending on various usability principles. In our thesis, we will create an understanding on the aspects of usability principles used to make a user interface more acceptable to the users. This understanding will lead future researches in the betterment of interface design of flight booking system.