Study aim: We aimed to assess the effect of high dose vitamin C on clinical improvement and inflammatory indices in moderate to severe COVID-19 patients. Design: Randomized double blind clinical trial on two arms containing control and treatment group Settings and conduct: After selecting eligible patients in Dr. Shariati teaching hospital, pharmacist will allocate each patient to one study group by table of randomization and prepare the serum content. investigator, physician and healthcare provider are blinded. biochemical tests, inflammatory indices and respiratory support will be assessed based on defined schedule. At the end of the study results will be presented to analyzer in two blinded groups. Participants/Inclusion and exclusion criteria: Confirmed moderate to severe COVID-19, age more than 18 years and consent for participation in the study, negative history for nephrolithiasis, chronic kidney disease (stage 4) and hemodialysis, pregnancy and breastfeeding will included in this study. Intervention groups: Treatment group will receive 12g Vitamin C in dextrose5% to total volume of 200 ml for 4 days and placebo group will receive distilled water in dextrose5% to total volume of 200 ml for 4 days. Main outcome variables: Time to clinical improvement (TTIC), change in Sequential Organ Failure Assessment (SOFA) score, National Early Warning Score (NEWS) 2, Ordinal scale for clinical improvement, Length of hospital stays, inflammatory indices, and mortality