Since the economic reform in 1990s, rapid urbanization took place in Ulaanbaatar, the capital city due to the increased influx of in-migrants from rural area of Mongolia, and the shortage of housing led informal unregistered residents reside in Ger district on the outskirts of the capital. The Ministry of Health has reformed health system to ensure universal access to primary healthcare (PH) in a form of family health clinic (FHC), but unregistered residents cannot utilize sufficient level of health services because they are not allowed to register the residence according to the population curb policy, and low health insurance coverage results in high out-of-pocket payment. Two-hundred unregistered and temporary residents in four khoroos in Sukhbaatar district were selected for the survey asking their health status, living condition, and the experience of visiting FHC. In addition, adequacy, readiness and availability of FHC were assessed. Also, barriers of access to PH for unregistered residents was asked to FHC service providers to triangulate the data through focus group discussion. More than a half of respondents reported that they are living in ger, and about 20% of respondents were diagnosed with chronic diseases such as hypertension and renal dysfunction. Respondents were aware of the PH services through FHC, but they showed low satisfaction and trust in FHC services. The largest barriers of visiting FHC were their residential status and the lack of health insurance. In terms of the service adequacy, individual FHC reported that it was 78.2%, 65.9%, 74.4% and 74.9%, and the issue of providers’ burnout and patients’ long waiting time would decrease the efficiency of service delivery. The readiness was 61.8%, 74.9%, 76.6% and 64.8%, so it means the capacity of service delivery should be improved by proper on-the-job training or refresher education. Lastly, the availability was as high as 87.4%, 91.6%, 78.9% and 85.5% each. Some FHC reported that they don’t accept unregistered residents to minimize the financial loss or to circumvent the unnecessary responsibility of misbehavior. The research revealed the limitation of privatized PH delivery and the fragmentation of health system, and unfinished business to achieve UHC in Mongolia.