Objective: To examine the factors associated with caregivers' burden in individuals providing care to family members suffering from serious mental illness. Methods: This Cross Sectional Study was carried out at Armed Forces Institute of Mental Health, Rawalpindi, from May 2015 to December 2015. A purposive sample of 120 family caregivers (60 males and 60 females, age range= 18-65) who were taking care of patients with serious mental illness (i.e. Major Depressive Disorder, Bipolar Disorder & Schizophrenia) for at least one year were recruited from the hospital and assessed through Zarit Burden Interview (ZBI) and Brief COPE inventory. The decline in functional status, and diminished physical capacity compromising the independent living of the care recipient was assessed through Katz Index of Independence in Activities of daily living (ADL) and Lawton Instrumental activities of daily living (IADL). Results: The results suggest that the longer the duration of illness (F=25.71, p < 0.01), with increased impairments of care-recipients, (decline in functional status, F=21.33, p < 0.001; diminished physical capacity F =32.41, p < 0.001) the more the burden experienced by the caregivers. Moreover, caregivers who were married (t=-2.98, p < 0.01), less educated (t =5.48, p < 0.01), lived in rural area (t = -7.99, p < 0.01), had lower monthly income (t = -4.95, p < 0.01) provide longer hours of caregiving (F=19.12, p < 0.001) and used avoidant coping behavior (F= 56.37, p < 0.001) reported significantly higher caregiver burden than caregivers who were unmarried, more educated, lived in urban area and had better income. Conclusion: The results of study demonstrate that caring for family members with serious mental illness impacts the caregivers' wellbeing. It, therefore, highlights the need for support and counseling services for the caregivers to reduce the burden of caring.