Objective: To find out the relationship between hematological parameters and disease severity among COVID-19 patients. Methods: This cross-sectional analytical study was conducted in the COVID unit of Dhaka Medical College and Hospital (DMCH). The study was conducted from October 2020 to September 2021. Patients diagnosed with COVID-19 infection detected by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) admitting in the Corona unit of DMCH, Dhaka, Bangladesh. Purposive sampling technique was applied to collect the samples. 96 patients were included for the study. Result: In case of severity of patients, 17 (17.7%) patients had mild illness, 29 (30.2%) had moderate illness and 50 (52.1%) patients had severe illness due to COVID-19 infection. Furthermore, according to gender, the proportion of male patients (70.0%) were significantly more in severe group compared to mild/moderate group (47.8%). Moreover, the mean age of the patients in severe group (56.2± 14.3) were significantly higher than the mean age of the patients in mild/moderate group (48.7±12.5) as p=0.007. Patients in severe group had significantly higher neutrophil count than mild/moderate group (p<0.001). On the other hand, patients in severe group had significantly lower lymphocyte, eosinophil and platelet count than mild/moderate group (p<0.05). The neutrophil-to-lymphocyte ratio (NLR) was significantly higher in severe group compared to mild/moderate group (p<0.001).