BACKGROUND: Post-operative complications of GI and HPB cases has been associated with significant morbidity, mortality and increased medical cost after underwent abdominal surgery in low preoperative serum albumin level. Patients underwent surgery developed adverse outcomes up to 63% with associated determining factors including preoperative serum albumin, post-operative serum albumin, and age representing a significant risk factors. Most of them needs aggressive nutritional optimization before discharge from the hospital which is widely accepted but not widely practiced in Ethiopia surgical care principle. OBJECTIVE: To assess the magnitude of post-operative complications of GI and HPB patients in different serum albumin level and associated factors in TGSH. METHODS AND MATERIALS: The study was conducted using a retrospective cross-sectional study in patients who were admitted and operated with GI and HPB cases that fulfill the inclusion criteria and were on follow up in TGSH surgical department of GI and HPB clinic from January 2018 to March 2021. Patients with GI and HPB cases were reviewed and patients with preoperative serum albumin level were analyzed in particular. The data was entered and analyzed using SPSS software version 26. Descriptive statistics like frequency tables and descriptive summaries were used to describe the variables. Binary logistic regression model was used to analyze the association between variables. Bi variable and multivariable logistic regression analysis were used and the results were presented in tables and charts. Odds ratio (OR) was used to compare associations between dependent and independent variables. RESULT: The overall magnitude of postoperative complication was 244(63.5%).it was varies with levels of serum albumin pre-operatively and post-operatively ,which were serum albumin of less than 3.5mg/dl were 64.6% and 74.2% and in normal pre-operative and post-operative serum albumin were 17.4% and 8.6% respectively. Conclusion and Recommendation: This study showed that pre-operative decrease of serum albumin level predicts postoperative adverse outcomes in patients undergoing major abdominal surgery of GI and HPB cases. In addition the present study concluded that the overall incidence of postoperative complications is higher than developed countries. So, more attention should be given to GI and HPB patients to be nutritionally optimized before schedule to be operated and recommended that further study on this topic with full nutritional assessment. Keywords: Magnitude, post-operative complications, associated factor, Ethiopia