Background: Although antiviral properties of vitamin D are recognized, the influence of parental Vit D supplementation on COVID-19 disease has not been determined. Objective: The aim of study was to evaluate impact of prompt treatment of Vit D deficiency on COVID-19 patients. Patients and Methods: A randomized controlled experiment was carried out on 250 COVID-19 patients. Patients were categorized into two cohorts: one cohort received daily intramuscular injection of 200,000 IU cholecalciferol for four consecutive days, while other cohort received daily oral dose of 10,000 IU cholecalciferol. The latter group functioned as control group. Before and after therapy, serum 25(OH)D level, inflammatory markers and electrolytes were measured, besides, clinical follow-up. Results: In Vit D group, the 25(OH)D levels considerably increased after 7 days compared to initial levels (32.48 ±9.64 Vs 13.77 ±6.51 ng/mL, respectively). All Vit D deficient patients have transitioned to sufficient status. Levels of markers (ESR 50.99±17.56 mm/hr, CRP 30.75 ±24 mg/L, and ferritin 392.05 ±139.17 ng/mL) decreased after seven days (29.74±8.97 mm/hr, 10.52 ±13 mg/L, and 94.59 ±27.14 ng/mL, respectively). A substantial clinical improvement occurred in Vit D group compared to their initial condition. Also Vit D deficiency was found to significantly increase risk of COVID-19 mortality by factor of 15.375 [AOR = 15.375, 95% CI: 1.898-124.52, p=0.01]. Conclusion: A daily intramuscular injection of 200,000 IU cholecalciferol for four consecutive days has been proven to significantly enhance clinico-labarotaory parameters in COVID-19 patients. Considering higher Vit D supplementation as a potential treatment for COVID-19 is a viable option.