Serum albumin and conventional BMI (cBMI) are commonly used indices of malnutrition status. Because cBMI does not reflect fluid accumulation, modified body mass index (mBMI, serum albumin × cBMI) is a more accurate measure of malnutrition status. This study investigated the association between preoperative mBMI and postoperative 1-year mortality, in comparison with serum albumin and cBMI. Medical records of 80,969 adult patients who underwent surgical procedures in a tertiary care hospital between 1 January, 2011 and 31 December, 2015 were retrospectively reviewed. Postoperative 1-year mortality increased with reduction in cBMI, mBMI, and albumin separately (P < 0.001). When considering interaction between cBMI and albumin, albumin was the only significant factor of postoperative 1-year mortality [odds ratio: 0.377, 95% confidence interval (0.245-0.579), P < 0.001], while cBMI or interaction (cBMI∗albumin) was not significant (P > 0.05). Adjusted area under the curve (AUC) was highest (0.885) in the overall model (cBMI + albumin + cBMI∗albumin); adjusted AUC between mBMI and albumin did not differ significantly (P = 0.558). Low albumin is the strongest independent predictor of postoperative 1-year mortality among the three variables (albumin, cBMI, mBMI). Adding cBMI to albumin does not increase the validity of the AUC of albumin.