Background/Aim: Clinical features of patients with COVID-19 have revealed a number of hematological markers associated with in-hospital mortality. In particular, the blood levels of white blood deferential including; lymphocyte, monocyte, and neutrophil counts. A growing evidence shows that ratios among lymphocytes, monocytes, and neutrophils are more accurate to predict mortality than cell count by itself l. For this reason, our main goal was to assess the negative clinical impacts prognostic performance of Monocytes to Lymphocytes ratio (MLR) compared with Monocytes and Lymphocytes counts in mechanically Ventilated Critically I'll COVID-19 Infected Patients.Methods: A retrospective study was conducted in our Royal Medical Services institutions for all mechanically ventilated critically I'll COVID-19 infected patients. An Independent T, Mann Whitney-U, and Chi Square Tests were used to analyze the parametric and non-parametric outcomes’ data. A receiver-operating characteristic was plotted to determine the area under the curve of each tested prognosticator and to identify the optimal cutoff point, sensitivity, specificity, Youden index, accuracy, and positive/negative predictive values.Results: The mean age of the whole study cohort was 58.37±9.96 years, and the Survivors Cohort were insignificantly older than the Non-Survivors Cohort (58.55±9.95 years versus 58.09±10.05 years, respectively, p>0.05). Significantly, males were distributed in the study in approximately a 2: 1 ratio compared to females. Overall 28-day ICU mortality was detected in 94 (48.70%) during an average of 12.40±4.79 days of ICU length of stay. Haematologically, the monocytes count and their ratio to lymphocytes count were significantly lower in the Survivors Cohort compared with Non-Survivors Cohort (1108±112 cells/µl and 0.25±0.10 versus 2550±421 cells/µl and 0.89±0.19, respectively, p<0.05). Conversely, the count of lymphocytes was significantly higher in the Survivors Cohort compared with Non-Survivors Cohort (4410±422 cells/µl versus and 2866±210 cells/µl, respectively, p<0.05). Conclusion: In this study, we concluded that the increasing of monocytes count and monocytes to lymphocytes ratio or decreasing lymphocytes count in COVID-19 infected patients, especially the mechanically ventilated critically ill cohort, could be potentially used for their reasonable performances and prediction utility in early identification and stratification of COVID-19 infected patient’s severities.