Background and Aim: Prevalence of mental disorders is growing worldwide so that, according to WHO's statistics, more than 0.5 million of the world population suffer from mild or acute mental diseases. Regarding the increasing feature of such diseases, it is obviously necessary to carry out a new study aiming at determining the prevalence of mental disorders in Birjand. Materials and Methods: This descriptive-analytical study, which employed multiple randomized cluster sampling, was conducted on 582 over 15 year olds in Birjand. For data collection, SCL-90-R questionnaire was used. The questionnaire was filled out two experts of the Health Center of the province, in addition to a researcher. The questioners/interviewers completed the questionnaires by referring to the residences of the individuals after obtaining their conscious written consents and justifying the aims of the project. The obtained data was fed into SPSS software (version 15) and analysed. Results: 7.1% of the studied individuals had serious mental disorders and 47.9% had significant mental problems. Prevalence of various mental disorders was as follows: obsessive– compulsive disorders 5.5%, paranoid disorders 5.5%, depression disorders 6%, somatoform disorders 3.7%, inter-personal sensitivity disorders 1.7%, psychotics disorders 1.2%, anxiety disorders 2.6% and phobic disorders 1.3%.M ental disorders were significantly higher in women compared to men. Conclusion: Considering the above results it is concluded that mental disorders have a relatively high prevalence in Birjand.