Patient selection: those receive major abdominal operation (estimated postopeartive ICU stay more than 7 days); agree and give their consent(by their surrogate)within 24 hours after admissionto SICU; those are less than 18 y/o, pregnant, history of allergy to esomeprazole or famotidine, already have GI bleding are excluded Randomized to 2 groups: (1) 1st group to receuve pantoprazole 40 mg iv bolus stat and then qd, (2)2nd group to receive famotidine 20 mg iv bolus stat and then q12h;prophylactically used for 7 days; estimated enrollment of 60 patients for each group Monitoring items: recording opeartion procedure and time; APACHE II score at baseline, CBC、CXR at basleine and qod, stool OB at baseline; NG drainage、sputum、stool character, ICU routine （TPR, BP）;ICU stay，mortality rate at 30 days; EGD perfomed according to decision of attending physician End points: apparant UGI bleeding（tarry stool, meatemesus, large amount(more than 60 ml) of coffee ground from NG、decrease of Hb more than 2g/dl and endoscopically proved lesion）, mortality; ventilator associated pneumonia: new and persistent hazziness in CXR & examination of tracheal aspirate, judged by chest specialist