Background: One of the most important issues to diagnose mental disorders is the use of independent tools with similar results. Documented history is an important tool to diagnose diseases at each stage. The first and fundamental step in diagnosis is a comprehensive clinical interview. Objectives: The current study aimed to investigate the concordance of mental disorders diagnosed by psychiatrist's clinical interviews and results of thematic apperception test (TAT) conducted by psychologists. Methods: It was a cross sectional descriptive-analytical study. The study population included the patients (male and female) admitted in psychiatry section of Dr. Fatemi hospital in Ardabil, Iran, during 2009-2010. Data were collected from the archive of Ardebil Dr. Fatemi hospital and about 1200 patients were studied. Demographic data of participants including age, gender, marital status, admission date, discharge date and type of insurance were gathered from the recorded documents. The initial diagnosis of psychologist using TAT test was compared with the final diagnosis recorded for the patient attended by physicians. Data were analyzed using SPSS software, ver. 16. Results: Out of 1,200 participants, 660 (55%) were male and 540 (45%) female; 150 (12.5%) subjects were under 25 years old and 284 (23.66%) were over 45 years old. Also, 650 subjects were illiterate and the rest were literate. The results of Chi-square test showed a significant difference between diagnoses of psychiatrist's clinical interviews with TAT results conducted by psychologists (P < 0.045). In spite of this difference, the results of TAT test were in relative concordance with psychiatrists' diagnoses for schizophrenia, depression, bipolar depression and anxiety disease. Conclusions: The findings showed a relative concordance between psychiatric interviews and psychological assessments in the clinical diagnosis of mental illness.