User fatigue is an important issue in interactive evolutionary computation (IEC). A user’s evaluation will guide the evolution in IEC and a tired user probably misleads the algorithm. Firstly, the impact of the user fatigue is given. Secondly, the cause of the user fatigue is discussed: he/she will have to keep rational, which means that the user will have to keep the consistency between his/her evaluation and preference. And the necessity for the user to keep rational is also analyzed, which will ensure IEC to converge to the user-most-satisfactory individuals. The study of user fatigue established necessary foundation for future research.