This study aimed to find out: User Satisfaction on the Quality of Service of The Library Faculty of Mathematics and Natural Sciences, Gadjah Mada University Library. Data were collected by using questionnaires as the main method, documentation and interview as a complementary method. Population taken are all users MIPA UGM Faculty Library in April 2010. Samples taken as many as 100 respondents using incidental sampling technique. The analysis used are Qualitative Analysis and Quantitative Descriptive Analysis. Based on the Quantitative Descriptive Analysis, the results obtained can be summarized as follows: The score of assessment criteria based on the average value of 3.64 overall results thus obtained can be categorized that user satisfaction on the quality of library services at the Faculty of Science, GMU is good. Of the 24 indicators under consideration are translated into five sub-variables, namely: tangible, reliability, Responsiveness, Assurance, Empathy. Of the five variables into the user satisfaction in choosing a service to the Faculty of Science, Gadjah Mada University Library which became the main consideration for the variable of tangible, the average value user satisfaction is 3.77, so based on the average value is good. For the dimensions of reliability, the average value user satisfaction is 3.59, so based on the average value is good. For dimensions of responsiveness, the average value user satisfaction is 3.65, so based on the average value is good. For security dimension, the average value user satisfaction is 3.64, so based on the average value is good. For the dimensions of empathy, the average user satisfaction is 3.58, so based On the average value is good.