Introduction: common mental disorders are the most common mental disorders worldwide. Globally the magnitude of common mental disorder is estimated to be 29%. Among these 322 million is attributed to depression. Unipolar depressive disorders alone lead to 12.15% of years lived with disability, and rank as the third leading contributor to the global burden of diseases. The risk is higher among the poor, homeless, the unemployed, persons with low education, victims of violence, migrants and refugees, indigenous populations, children and adolescents, abused women and the neglected elderly.

Objective: the objective of this study was to assess the prevalence of common mental disorders and associated factors in Benchi-Maji zone, south west Ethiopia, 2017/2018.

Methods: A community based cross-sectional study was conducted in Bench Maji Zone from February 1^st, 2018 to May 1^st, 2018. A total of 1026 respondents were employed using multi stage sampling technique. Data was collected using the pre designed tools like Oslo social support scale and self-reporting questionnaire version 20. Data was entered using Epi data version 3.1 and transferred to statistical package for social science version 21 for analysis. Bivariate and multivariate logistic regressions were employed and odds ratio with 95% confidence interval was used to assess degree of association between variables. P value of less than 0.05 will be considered as statistically significant.

Result: Among 1,026 respondents 193 (18.82%) of them full fill criteria of common mental disorder. Factors like: family history of mental illness [AOR= 0.58, 95% CI (0.35, 0.97)], current use of tobacco [AOR= 0.45, 95% CI (0.24, 0.84)] and social support [AOR= 2.75, 95% CI (1.35, 5.37)], were significantly associated with common mental disorder.

Conclusion and recommendation:

This study revealed that prevalence of common mental disorder found to be high when it is seen from Ethiopian context as developing nation. Family history of mental illness, social support, and current tobacco use are significantly associated with common mental disorder.

And Benchi-Maji zone Health Beauro should give special consideration for individuals who has family history of mental illness during outreach activities.

Key words: common mental disorders, factors, Ethiopia