Background: Burden of caregivers of people with mental illness (PWMI) is considered to be a negative impact of the care provided by the family to the patient. However, little is known about the extent of the burden among caregivers of PWMI in Ethiopia. The aim of this study, therefore, is to assess the magnitude and associated factors of burden among caregivers of PWMI at Jimma University Medical Center, 2017.; Methods: Institution-based cross-sectional study design was employed among 406 conveniently selected caregivers of PWMI and interviewed using a structured questionnaire. Family burden interview schedule (FBIS) was used to assess burden of caregivers. Bivariate and multivariable linear regression analyses were performed to determine the predictors of burden among caregivers.; Results: Nearly two-thirds [264 (65.0%)] of the participants were male with a mean age of 38.45 ± 12.03 years. The mean score for burden among caregivers on family burden interview schedule was 23.00 ± 10.71. Age of the caregivers (β = 0.18, p < 0.001), being female caregiver (β = 2.68, p < 0.01), duration of contact hours with the patient per day (β = 0.74, p < 0.001), perceived stigma by the caregiver (β = 0.47, p < 0.001), and providing care for patients who had history of substance use in life (β = 1.52, p < 0.05) were positive predictors of higher burden among caregivers. Whereas, caregivers' income (β = 7.25, p < 0.001), caregivers who had no formal education (β = 4.65, p < 0.01), and caregivers' social support (β = 0.78, p < 0.001) were negatively associated with higher burden among caregiver.; Conclusion: Caregivers of people with mental illness experience enormous burden during providing care for their relatives with mental illness. Therefore, creating community awareness and targeted interventions in the area of treatment access, stigma, financial, and other social support for people with mental illness and their caregivers would help out to reduce these burdens.