Abstract. Objective: Training the librarians may be effective in their gaining experience and enhancing their performance. And by having a strategic development plan, it can also guarantee the survival and perfection of the organization. The present study aims to investigate the role of in-service training courses for librarians in their efficiency, in the Institute for the Intellectual Development of Children and Young Adults (IIDCYA).Method: The current study is an applied research which was conducted using the descriptive survey research method.A self-made questionnaire was used for gathering information. This questionnaire included 32 questions on Likert Spectrum Scale. The reliability of the questionnaire was confirmed based on Cronbach's alpha coefficient (0.981), in preliminary studies. The statistical population consists of 117 librarians of the Institute for the Intellectual Development of Children and Young Adults (IIDCYA). The analysis of data was conducted using Lisrel 8.5 and SPSS 22 depending on the kind of variables.Findings: The findings indicated that the mean of variables including coordination of courses with training needs, the magnitude of meeting the needs by the courses, being applicative of the course content and the effectiveness of courses on librarians' performances were 23.3, 42.3, 14.3 and 82.2, respectively, which were evaluated to be at moderate levels. The mean of other variables like the increase in skills after the courses, the efficiency of courses for librarians, using electronic technology by librarians were 37.3, 26.3 and 67.3, respectively, which were evaluated to be at high levels. Also, there is a significant relationship between in-service training courses and the increase in efficiency according to the Pearson's correlation coefficient (ρ = 0.770). Furthermore, there is a significant relationship between evaluating the in-service training courses and the increase in the knowledge and information of librarians (ρ = 0.849). So the hypotheses of the study are confirmed.Conclusion: The in-service training courses are effective on the efficiency of the librarians of the Institute for the Intellectual Development of Children and Young Adults (IIDCYA). Meanwhile by increasing the applied content of the training courses and more exact evaluation of effectiveness, their efficiency increases more. And at the end of this thesis, some recommendations are presented based on findings, in order to enhance the performance at the present and in the future.