Background: Coronavirus disease (COVID-19) is an infectious disease caused by a recently discovered coronavirus. Blood test including complete blood count is crucial in diagnosing of several viral and bacterial infection. Aims: This study aimed to assess the association between lymphocyte ratio and other WBC types and severity of COVID-19 pneumonia. Methods: The design of this study was a cross-sectional study. A complete blood count and erythrocyte sedimentation rate (ESR) was done for one hundred twenty-six COVID-19 patients (76 males and 50 females; aged 20–70 years). Patients were randomly recruited from multicenter in Al-Najaf Governorate, Iraq. Results: The study had revealed an inverse correlation between severity of COVID-19 infection and both lymphocytes and monocytes ratio even in patients with normal WBC count. Additionally, there was a direct correlation between platelets and leukocyte count. The relation between leukocyte count and ESR level was significant in a patient with elevated WBC only. Conclusion: Lymphocytes and monocyte ratios inpatient with COVID-19 infection can be used as predictors for the severity of infection. Increased leukocyte count resulted in increases in platelets inpatient with COVID-19.