The aim of this study is to investigate the effect of N-Acetyl cysteine infusion on improvement of liver and kidney function tests after bypass surgery in patients with obstructive jaundice. 30 patients referred with a diagnosis of obstructive jaundice to Imam Reza hospital, Tabriz, Iran, from March 2106 until the end of september 2016 who need a Bypass surgery (including all cases of cholecystojejunostomy, choledochodeudenostomy or choledochojejunostomy) are recruited for this study and randomly assigned into two groups. For the study group, besides the standard supporting treatments, immediately after the surgery N-acetylcysteine infusion will be started by a dose of 200mg/kg for the first 8 hours and then infusion of a dose of 100 mg/kg will be continued for the next 16 hours. The sameorder will be repeated in the second 24 hours. Primary outcome measures are serum bilirubin (direct, indirect and total), liver enzymes (ALT, AST), alkaline phosphatase, Gamma glutamyl transferase, creatinine during hospitalization on days 1, 3, 5, 7, 14, 21 and 30 after the surgery and secondary outcomes will be death, need for inotropes, need for dialysis, need for mechanical ventilation, pneumonia, liver encephalopathy and need for liver transplantation.