Abstract Background and Aims Intensive care patients had higher mortality and SOFA score is used for mortality prediction. The SOFA score was developed more than 25 years ago to monitor organ dysfunction. Nowadays, blood lactate levels indicate septic shock and work as a new vital sign. This study aimed to evaluate the blood lactate levels for prediction mortality and search for the best predictors for mortality. Method Physionet data (MIMIC3) were used for this retrospective analysis. A total of 13,357 intensive care unit patients were enrolled, they were admitted to the intensive care unit over 24 hours and aged 20 years old, were initially checked for blood lactate levels. We also analyzed 2,748 patients who were checked for blood lactate levels at baseline, the second day, and the third day. We classified patients into three groups according to initial blood lactate levels (group 0: <4 mmol/L, group 1: 4-7.99 mmol/L, group 2: >8 mmol/L). Results The total mortality rate (60.3% vs. 24.4%), 7-day mortality rate, and 30-day mortality rate were the highest in group 2 compared to group 0 among 13,357 intensive care patients. Charlson-comorbidity index, SOFA score, and APACHE III score were the highest in group 2 compared to group 0 among 13357 intensive care patients. The arterial pH, pC02, systolic blood pressure, and GFR were the lowest in group 2 compared to group 0 among 13357 intensive care patients. These findings were similarly found in 2,748 intensive care patients. The third-day blood lactate levels and APACHE III score showed a higher area under the ROC curves predicting total mortality, 7-day mortality, and 30-day mortality than initial blood lactate levels and SOFA score. The third-day blood lactate levels, third-day/initial blood lactate ratio, and APACHE III score were independently associated with total mortality, seven-day and 30-day mortality in 2,748 intensive care patients, but GFR, ventilator care and SOFA score were not. Conclusion The third-day blood lactate levels and APACHE III score were the best predictors for mortality in intensive care unit patients. Therefore, intensive care for reducing blood lactate levels is important to increase survival.