Objective: To find diagnostic precision of APACHE II score in predicting mortality in poly Trauma patients within first 24 hours of hospitalization. Study Design: Cross Sectional study. Setting: Emergency Department of Lahore General Hospital. Period: 2018-2019. Materials & Methods: A total of 270 patients who fulfilled selection criteria were enrolled in the study. To calculate APACHE II score, age, vitals, CBC level, Glasgow coma scale score and chronic health points were measured. Patients were classified as per their APACHE II score. After calculating APACHE II score patients were managed according to trauma severity and followed up till 24 hours to note the mortality. Data was analyzed in SPSS v. 20. Results: The mean age of patients was 38.53 ± 11.67 years with 173(63.91%) male and 97(36.09%) were female patients. Out of 270 cases, in hospital mortality occurred in 99(36.5%) while other 171(63.5%) were alive within 24 hours of admission. According to APACHE II score, 99(36.5%) cases had > 11.5 score and rests of 171(63.5%) had APACHE II ≤ 11.5. The sensitivity, specificity, PPV, NPV and diagnostic accuracy of APACHE II was 89.16%, 93.2%, 88.1%, 93.84% and 91.74%. Conclusion: According to this study, high accuracy of APACHE II for prediction of in-hospital mortality with high sensitivity, specificity, PPV, NPV and diagnostic accuracy as 89.16%, 93.2%, 88.1%, 93.84% and 91.74%. Using APACHE II in future we can devise an efficient treatment plan for poly trauma patients to reduce the probability of hospital mortality.