Splenectomy is an effective technique in living donor liver transplantation (LDLT) with small-for-size (SFS) liver grafts for overcoming SFS liver graft injury. However, the protective mechanism of splenectomy is still unclear. The aim of this study was to investigate how splenectomy could attenuate SFS graft injury through the measurement of biochemical factors, particularly the expression of endothelin (ET)-1, which is a key molecule of microcirculatory disorders by mediating sinusoidal vasoconstriction. We performed rat orthotopic liver transplantation using SFS liver grafts with or without splenectomy. We investigated intragraft expression of ET-1 mRNA and hepatic protein levels of ET-1. In addition, portal pressure, hepatic injury and morphological changes, and survival rate were evaluated. In result, intragraft ET-1 mRNA expression after SFS liver transplantation was significantly downregulated by splenectomy, and hepatic expression of ET-1 in SFS grafts was rarely observed. Splenectomy inhibited the increase in portal pressure, ameliorated SFS liver graft injury and improved the graft survival rate after SFS liver transplantation. In conclusion, splenectomy improved the SFS liver injury and decreased the expression of ET-1 by attenuating portal hypertension on SFS liver transplantation. Downregulation of intragraft ET-1 expression plays important roles in the protective mechanism of splenectomy in SFS liver transplantation.