Objective: This animal experiment investigated the donor-recipient liver size match for safe liver transplantation. Background: In spite of refinements in surgical techniques in reduced liver transplantation, the liver size disparity remains one of the most common complications in pediatric patients. Optimal size matching remains unknown. Methods: The experiment compared eight groups of liver-transplanted rats with designated ratios of donor and recipient liver weights. Donor livers harvested from rats weighing 420-520 g were reduced to the designated size by liver lobectomy and implanted in rats weighing 170-240 g. Bile secretion and serum aspartate aminotransferase (AST) activities in groups 2, 4, and 6 were studied after surgery. Results: Stepwise increase of the ratio of donor and recipient liver weights from 1.04:1 in group 4 to 1.26:1 in group 3, 1.56:1 in group 2, and 2.04:1 in group 1 caused stepwise decrease of survival rates from 83.3% to 66.7%, 16.7%, and 0%, respectively. Stepwise decrease of the ratio from 1.04:1 in group 4 to 0.79:1 in group 5, 0.53:1 in group 6, 0.35:1 in group 7, and 0.24:1 in group 8 also caused stepwise reduction of survival rates from 83.3% to 66.7%, 50%, 0%, and 0% in each group. Conclusion: The range of ratios of donor and recipient liver weights for successful rat liver transplantation is from 0.53:1 to 1.26:1. Increase and decrease of ratios of donor-recipient liver weights from equal size do not increase the recipient survival rates. Recipients of reduced donor liver weights tend to have a higher survival rate than recipients of increased donor liver weights.