Objective: From January 2010 to December 2018, 788 cases of non-sensitized living donor kidney transplant recipient and donor pairs were enrolled. The subjects were divided into four groups according to the relative size of kidney and pre-transplant DM status: non-DM large kidney, non-DM small kidney, DM large kidney, and DM small kidney. We compared allograft outcomes between these four groups. Methods: The four groups did not show differences in the development of de novo donor-specific antibody and acute rejection. However, a significantly greater decline of allograft function and increased proteinuria were observed in the DM small kidney group than in the other groups. The highest death-censored graft loss (DCGL) rate (P = 0.008) was observed in this group and the combination of relatively small kidney size and pre-transplant DM was an independent risk factor for DCGL. In addition, the relatively small kidney and pre-transplant DM showed significant interaction with each other. Results: The mismatch between donated kidney volume and recipient body size should be considered in donor selection of patients with pre-transplant DM. Conclusions: Objective: The aim of this study was to investigate allograft outcomes when relatively small kidneys were donated to patients with pre-transplant diabetes mellitus (DM). Methods: From January 2010 to December 2018, 788 cases of non-sensitized living donor kidney transplant recipient and donor pairs were enrolled. The subjects were divided into four groups according to the relative size of kidney and pre-transplant DM status: non-DM large kidney, non-DM small kidney, DM large kidney, and DM small kidney. We compared allograft outcomes between these four groups. Results: The four groups did not show differences in the development of de novo donor-specific antibody and acute rejection. However, a significantly greater decline of allograft function and increased proteinuria were observed in the DM small kidney group than in the other groups. The highest death-censored graft loss (DCGL) rate (P = 0.008) was observed in this group and the combination of relatively small kidney size and pre-transplant DM was an independent risk factor for DCGL. In addition, the relatively small kidney and pre-transplant DM showed significant interaction with each other. Conclusions: The mismatch between donated kidney volume and recipient body size should be considered in donor selection of patients with pre-transplant DM.