The barriers and facilitating factors for integrating mental health into primary health care have been well documented in the literature, but little is known about the perspectives of primary health care workers (who provide integrated mental health care) on barriers and facilitating factors of the health system for scaling up mental health interventions in low and middle income countries. This study aimed to explore these perspectives of primary health care workers within the health system, and identify possible strategies to optimize the integration of mental health in primary health care.The study was conducted in the Chitwan district of Nepal with 55 purposively selected primary health care workers representing prescribers (N = 35), non-prescribers (N = 12) and Female Community Health Volunteers (N = 8). Using a semi-structured interview guide, experienced qualitative researchers collected data between September 2016 and May 2017. The interviews were audio-taped, transcribed and then translated into English. The transcripts were coded using Nvivo 10 software and themes were generated for the thematic analysis.According to the health workers, the facilitating factors for scaling up mental health services in primary health care setting in Nepal included; (1) availability of guidelines, protocols and awareness raising materials, (2) provision of supervision, (3) referral systems being in place, (4) patient record keeping, (5) community sensitizations and home visits, and (6) provision of psychosocial counseling. The barriers identified included; (1) shortage of psychotropic medicines, (2) lack of private space for counseling, (3) workload and health workers' grievances regarding incentives, and (4) perceived stigma causing dropouts.The findings suggest that implementation of mental health services through primary health care workers in resource-poor setting is possible when health system level barriers are addressed and facilitating factors are strengthened. In order to address these barriers the health workers suggested a few strategies which included; ensuring dedicated staff available at health facility, allocating dedicated and confidential space for counseling, improving on incentives and motivational benefits to existing health staff, organizing policy level advocacy for mental health, improving medicine supply chain management and strengthening systems for supervision, referral and mental health information management.