Background and objectives: Coronavirus disease 2019 (COVID-19) is affecting millions of people world-wide. It is caused by the severe acute respiratory syndrome corona virus 2(SARS-CoV-2). The laboratory findings are very important to assess the progress of the disease. The present study is aimed to discuss the biochemical parameters among mild, moderate and severe COVID-19 patients. Materials and methods: A cross sectional study were conducted in the Department of Physiology, Dhaka Medical College, Dhaka from January 2020 to December 2020. After obtaining ethical clearance, a total of 100 real time-polymerase chain reaction (RT-PCR) COVID-19 positive patients were selected from Dhaka Medical College Hospital. With all aseptic precautions, 10 ml of venous blood was collected from ante-cubital vein. D-dimer, prothrombin time, C-reactive protein (CRP), lactate dehydrogenase (LDH), serum ferritin, random blood glucose (RBG), serum creatinine, serum glutamic-pyruvic-transaminase (SGPT) and serum albumin measured in the Department of Laboratory Medicine, Dhaka Medical College Hospital, Dhaka. CRP was measured by Immunoturbidimetric method, serum ferritin was estimated by Chemiluminescent microparticle immunoassay. STA-neoplastine CI plus used with STA-R analyzer was used for determination of prothrombin time. D-dimer was estimated by Immunofluorescence Assay method. Serum LDH is measured by Dimention clinical chemistry system, serum albumin is measured by bromocresol purple dye binding method, serum creatinine is measured by Jaffe alkaline picrate method and serum SGPT is measured by colourmetric (IFCC 1980) method and RBG is measured by enzymatic colorimetric method (GOD-PAP). Data were recorded in a pre-designed structured data collection form. For statistical analysis, ANOVA followed by Bonferroni test, Chi square test, Spearman’s rho correlation coefficient test was performed as applicable using SPSS for windows version 25.0. Results: By analyzing biochemical parameters of mild, moderate and severe RT-PCR positive 100 COVID-19 patients revealed evaluation of biochemical parameters shows severity of the disease was significantly associated with CRP, SGPT, S. Creatinine, LDH, Ferritin, D-dimer & Prothrombin time. No significant association was found with RBG & S. Albumin. Bonferroni correction following ANOVA was performed to compare between each group. Spearman’s correlation reveals statistically significant strong positive correlation with CRP, Ferritin & D-dimer, moderate positive correlation with S. Creatinine, LDH and mild positive correlation with SGPT & Prothrombin time. Conclusion: This study showed D-dimer, prothrombin time, CRP, LDH, ferritin, serum creatinine and SGPT are significantly associated with the severity of the illness that is higher in severe group in comparison to mild and moderate groups. So, comprehensive analysis of the biochemical parameters will be very helpful for early identification & better management of severe disease.