The objectives of this research “Assessment of Service Quality at Central Library of King Mongkut’s University of Technology North Bangkok” were to evaluate of service quality, identify the dimensions that determine the customer’s evaluation of service quality, problems and suggestions in using the library and the essential attributes that library should allocate for good service quality by using the concept of The Zone of Tolerance. The findings revealed that user responses for each level of service show that the perception of service performance and the minimum service expectation were moderate, and the desired service expectations were heighten to moderate. The perception of service performance and the minimum service expectation at the highest score was the atmosphere of the library that can encourage the research purpose. The highest desired service expectation was related to electronic resources’ needs. The result of The Zone of Tolerance revealed that there were library’s services which were not satisfied user such as teaching or advising user or opening the advised service class, understanding user’s needs, creating user’s confident, providing the thorough information or services for user, supporting user instantly and giving the clearly solution when user have got problems. The greatest problem faced by users was that there were not enough books and some of books which the library were out of date