Shock index (SI) is easily measured in prehospital field and is useful to predict mortality in injured patients. We assessed the association between altered stability of SI between Prehospital to Inhospital Period and Mortality during ED stay in severe trauma patients using emergency medical services (EMS). This is prospective observation study using EMS Severe Trauma Database in South Korea between Jan and Dec 2011. Prehospital SI (PSI) was measured by EMS provider at the scene. Hospital SI (HSI) was assessed from initially measured vital signs at ED. We categorized alteration of prehospital and inhospital SI by 4 groups; stable PSI and HSI (SPSH), unstable PSI and stable HIS (UPSH), stable PSI and unstable HIS (SPUH), unstable PSI and HIS (UPUH). Primary outcome was mortality during ED stay. During the study periods, 9,668 cases with severe trauma was registered into EMS ST DB. Among them, the value of PSI was 1.2±6.2 and HSI was 0.9±1.2. Case numbers of SPSH, UPSH, SPUH and UPUH was 3,290 (34.0%), 2,345 (24.3%), 1,128 (11.7%), and 2,905 (30.1%), respectively. Compared by SPSH group, odds ratio for ED mortality of UPSH, SPUH, UPUH was 1.97 (95% CI: 1.03-3.77), 7.79 (4.36-13.9), 119.7 (67.3-182.3), respectively. Alteration of stability of SI between prehospital to inhospital period was associated with higher ED Mortality in trauma patients using EMS.