Background: The treatment of patients with COVID-19 included supportive care to relief the symptoms mainly. Although WHO mentioned there is not any effective treatments for COVID-19, but there is some reporting about the use of some antiviral drugs. The aim of current study is to determine the effect of Arbidol (ARB) on COVID-19 disease. Methods: Using an open label randomized controlled trial, effectiveness of ARB on COVID-19 disease was conducted in a teaching hospital. One hundred eligible patients with diagnosis of Covid-19 recruited in the study and assigned randomly to two groups of either Hydroxychloroquine followed by Kaletra (Lopinavir-ritonavir) or Hydroxychloroquine followed by ARB. The primary outcome was hospitalization duration and clinical improvement 7 days after admission. The criteria of improvement were relief of cough, dyspnea and fever. Time to relieving fever was assessed across two groups too. Without any drop out, 100 patients were entered to final analysis with significant level of 0.05. Results: The mean age of the patients was 56.6 (17.8) and 56.2 (14.8) in ARB and Kaletra groups respectively. Majority of patients were male across two groups (66% and 54%). The duration of hospitalization in ARB group was less than Kaletra arm significantly (7.2 versus 9.6 days; P=0.02). Time to relief fever was almost similar across two groups (2.7 versus 3.1 days in ARB and Kaletra arms respectively). Peripheral oxygen saturation rate was different after seven days of admission across two groups significantly (94% versus 92% in ARB and Kaletra groups respectively) (P=0.02). Based on multiple linear regression analysis, IHD, Na level and oxygen saturation at the time of admission and type of therapy were the independent adjusted variables that determined the duration of hospitalization in patients with COVID-19.Conclusion: Our findings showed that Arbidol, compared to Kaletra, significantly contributes to clinical and laboratory improvements, including peripheral oxygen saturation, requiring ICU admissions, duration of hospitalization, chest CT involvements, WBC, and ESR. We suggest further studies on ARB using larger sample size and multicenter design.Trial registration: IRCT20180725040596N2 on 18 April 2020.