Thailand has put much effort in registering undocumented/illegal migrants/aliens and protecting health of non-Thai populations for years. One of the most distinct policies in protecting health of the non-Thais is the establishment of public insurance schemes, for instance, 'Health Insurance for People with Citizenship Problems' (HI-PCP) and 'Health Insurance Card Scheme for cross-border migrants' (HICS). However, in practice, there are always people failing to be enrolled in the insurance schemes due to various reasons, such as poor law enforcement and fear of being exposedby state officials amongst undocumented migrants/aliens. These uninsured migrants/aliens often visited state-run health facilities when they experienced severe illnesses. Some cannot afford the treatment cost. Accordingly, the visited facilities inevitably shoulder this unpaid debt instead. This research therefore sought to explore the reimbursement system for subsidising treatment cost for uninsured migrants/aliens with critical illnesses in Thailand. Mixed-methods design was employed. The study comprised three objectives. The first objective aimed to review literature regarding international experience in managing care for undocumented migrants/aliens. The second objective used qualitative approach (in-depth interviews and focus group discussion) to investigate perspectives of healthcare staff in establishing the reimbursement system of recouping cost of treating uninsured migrants/aliens. The third objective employed quantitative method to estimate budget impact for the whole country in case that the Thai government implements such a system.  The review in the first objective found that level of care for undocumented migrants/aliens from international experience could be classified into four levels: level 1—insurance for fully legalised migrants, level 2—insurance for illegal or undocumented migrants who passed the registration process arranged by the state, level 3—reimbursementscheme of specific diseases, and level 4—contingency/special fund. Compared to the Thai context, the Social Security Scheme might be regarded as level 1, the HICS as level 2, disease control programmes by the Department of Disease Control as level 3, and the contingency fund by the National Health Security Office as level 4. However, level-3 and level-4 care programmes were normally managed on ad hoc basis and have not been incorporated into the main insurance schemes. In terms of providers' views, most interviewees opined that the proposed reimbursement system is useful to health facilities. Some interviewees suggested that the system should cover not only the cost of treatment, but also health promotion and disease prevention activities. The proposed system should be set up in parallel with the improvement of the HICS administrative performance. Nonetheless, there were some concerns raised by the interviewees. Firstly, if this policy is clearly announced as though it is rights for undocumented/illegal migrants, this might indirectly encourage existing migrants to avoid the purchase of health insurance card. Secondly, this policy did not tackle the deep rooted problems regarding undocumented/illegal migrants. The bottom line was that migrant policies in Thailand at the status quo could not effectively prevent illegal border crossings. Lastly, the source of funding has not been explored. Should the reimbursement system reply on additional earmarked central budget, one might argue that such a proposal is taking advantage of Thai tax-payers. If this budget is drawn from the HICS, there might be less political constraint. Another option that is worth considering is using non-health budget, such as extracting part of the visa fee for tourists. Concerning quantitative analysis, the estimated budget is approximately 149 to 891 million Baht, depending on the scope of services and nationalities of the users (whether the reimbursement system should cover Cambodian, Laotian, and Burmese migrants only, or cover all ethnic groups). This study experienced some limitations. Firstly, it is difficult to ensure correctness of utilization data as they are routine hospital-based records. Secondly,  many figures used in the analysis might be out of date and based on several assumptions. Thirdly, service users (patients) were not included in the in-depth interviews. Lastly, this research has not explored sources of funding in detail. Key policy recommendation is the Thai government should set aside some budget for recouping cost of the treatment for uninsured migrants/aliens at public health facilities for around 149 to 891 million Baht. The size of budget depends on the openness of policy for migrants. The government might start from migrants from Cambodia, Lao PDR, and Myanmar first, then gradually expand to other non-Thai populations. The reimbursement system should be introduced in tandem with the improvement of the HICS administrative performance.