Objective To establish different kinds of reduced size liver transplantation model of rats， and to explorethe optimal marginal size of liver graft in orthotopic liver transplantation， in purpose of providing a kind of animal modelfor the study about mechanism and prevention measures of small-for-size syndrome. Methods One hundred and ninety-two rats were randomly divided into whole liver graft transplantation group （underwent whole liver graft transplantation），half liver graft transplantation group （the median lobe and right lobe of the liver were selected to be the graft）， small size liver graft transplantation group （the median lobe of the liver was selected to be the graft）， and extra-small size liver graft transplantation group （the median lobe and left lobe of the liver were reduced， and remained lobes were selected to be the graft）， each group enrolled 48 rats. After liver graft transplantation， 24 rats of each group were selected to observe the survival situation， 12 rats of each group were selected to measure portal venous pressure at time point of before operation，and 5， 15， 30， 45， and 60 minutes after transplantation. The other 12 rats of each group were test the level of alanine aminotransferase （ALT）. Results Seven-day survival rate of the whole liver graft transplantation group， half liver graft transplantation group， small size liver graft transplantation group， and extra-small size liver graft transplantation group was 100% （24/24）， 87.5% （21/24）， 37.5% （9/24）， and 0 respectively. Portal venous pressure of whole liver graft trans-plantation group was stable after opening the portal vein， although there was slight increase at prophase in half liver graft transplantation group， and then the portal venous pressure would let down， keeping stable at the later stage. But in small size liver graft transplantation group and extra-small size liver graft transplantation group， the portal venous pressure incr-eased and got the top at 15 minutes after opening the portal vein， and then induced， keeping stable during the 45-60 minutes.Portal venous pressure at the point of 5 （r=-0.942）， 15 （r=-0.947）， 30 （r=-0.900）， 45 （r=-0.825）， and 60 （r=-0.705）minutes after opening the portal vein were significantly related to liver graft size （P＜0.001）. The levels of ALT in wholeliver graft transplantation group and half liver graft transplantation group were both lower than those of small size livergraft transplantation group and extra-small size liver graft transplantation group （P＜0.05）， and levels of ALT in small size liver graft transplantation group was lower than extra-small size liver graft transplantation group too （P＜0.05）. Levelof ALT at 24 hours after transplantation were significantly related to liver graft size （r=-0.685， P＜0.001）. Conclusions The minimum graft volume/standard liver volume （GV/SLV） in reduced size liver transplantation in rat is 50%. The liver graft whose GV/SLV is 30%-35% should be considered as marginal size liver graft， and the liver graft whose GV/SLV less than 30% should be considered as extra-small size liver graft in the rat.