Severely immunodeficient mouse strains, such as NOD.Cg-Prkdcscid Il2rg-/-, have been used to study the immune response in humans. These mice lack T, B and natural killer (NK) cells and allow for engraftment of human peripheral blood mononuclear cells (PBMCs). However, hPBMC engraftment may lead to severe acute xenogeneic graft versus host disease (GvHD) which arises due to a destructive response from engrafted human T cells interacting with host APCs that express major histocompatibility complex (MHC) class I and II. GvHD shortens animal life span and results in only a brief window of evaluation of human immune cell functions and therapeutic efficacy in such mouse models. Efforts have been undertaken to reduce human PBMC-induced GvHD and to subsequently extend the window of human immunity functional assessment by eliminating mouse MHC I and MHC II. Beta-2-microglobulin (B2M) knockout mice present MHC I functional deficiency with an unintended result of shorter antibody half-life because B2M is also associated with neonatal Fc receptor (FcRn), which is important for antibody recycling and retention. To overcome this limitation, we engineered B-NDG B2M knockout mice plus (NOD.Cg-Prkdcscid Il2rg-/-/Bcgen) that is deficient in MHC I expression but expresses the B2M gene fused in the FcRn gene to remain FcRn-mediated antibody metabolism. In the present study we demonstrate that B-NDG B2M KO mice Plus showed minimized and delayed onset of GvHD compared to B-NDG mice. Human CD45+ cells and CD8/ Treg ratio were increased over the several weeks of post human PBMC engraftment, suggesting that the reduced GvHD is related to the sufficient MHC I deficiency due to B2M KO in the mice, and the increased CD8/Treg cell population could be beneficial for the study in human T-cell mediated antitumoral therapy. In conclusion, B-NDG B2M KO mice Plus is a well-validated mouse model for the studies with hPBMC engraftment, which is characterized with minimized GvHD and enhanced CD8/Treg population.