Objective: COVID-۱۹ has presented numerous epidemiological and clinical pictures from its beginning and much effort has been paid to detect the behavior of disease and its new types. Therefore, in this study, we aimed to compare the in-hospital survival time of Delta and Omicron variant patients admitted to the intensive care unit. Methods: This was a secondary data analysis of the QCOVICU data registry of ۲۰۰ COVID-۱۹ patients admitted to the ICU of Shahid Beheshti-Amir Al-Momenin Hospital of Qom City, in ۲۰۲۱. Likewise, time to event data, demographics, and baseline laboratory data was collected. Time of transfer to ICU, survivals, and possible predictors of hazards of death was compared within the variants of Omicron and delta.Results: Two hundred patients (۶۲.۹۸±۱۹.۹۴ years old, ۹۴ females/۱۰۶ males; ۱۰۰ Delta and ۱۰۰ Omicron variant) participated in this study. Fifty percent of the population had died. Cross-tabulation showed comparable death rates among variants of delta and omicron (۵۰.۵% vs. ۵۱%; p=۰.۹۹۹). There was a statistically significant higher time to ICU admission in Delta variant victims than in Omicron variant victims. The mean survival time of delta variant patients was ۲۱.۵۲ days (۹۵% CI: ۱۷.۹۶ – ۲۵.۰۹) which was statistically higher than the mean survival of omicron patients (۱۷.۱۵ days, ۹۵% CI: ۱۳.۶۵-۲۰.۶۴, p=۰.۰۱۸). The mean survival time of delta variant patients was statistically higher than omicron patients (۲۱.۵۲ vs. ۱۷.۱۵ days, p=۰.۰۱۸). Gender, age (years), and lymphocyte count were significant predictors of mortality based on the Cox regression analysis (P>۰.۰۵). There was a ۵.۹ times higher risk of mortality in females compared with males’ gender after adjusting for other variables and a ۵.۶% increase in death risk with a ۱-year increase in age, and a ۳۱.۸% decrease in death risk with a ۱% lymphocyte percentage increase.Conclusion: Critically patients with Delta variant are getting ICU admitted later and withstand more days at ICU than Omicron patients. It seems that Omicron variant causes sudden deterioration of the patient's condition.