The purpose of this study is to identify if the design principles control, consistency and error prevention affect the usability during the booking process on travel agencies websites, whether they are implemented or not. The research methodology consists of qualitative and quantitative methods such as usability testing and a quick quantified questionnaire, the System Usability Scale model. To identify the design principles' effect on usability, this study will examine three design principles; control, consistency, and error prevention, through usability aspects; learnability, efficiency, and satisfaction. Observations and a think aloud method have been used to better understand the users perception of the website, whether they feel safe and in control during their interaction with the system. The study also examines how well the user recover from an error or how well the system has prevented potential errors from occurring. The System Usability Scale model clarifies the users thoughts and perception of the website, where interpretation is being excluded with a structural and quantified outcome. The study can conclude that usability is dependent on the design principles that has been examined, particularly for the specified context on travel agencies websites. The study also shows that the information architecture on the examined websites has an overall positive user experience and usability, that only need minimal improvements.