Purpose: Novel coronavirus disease (COVID-19) is the current global concern. Radiotherapy (RT), commonly employed in cancer management, has been considered one of the potential treatments for COVID-19 pneumonia. Here, we present the final report of the pilot trial evaluating the efficacy and safety of low-dose whole-lung irradiation (LD-WLI) in patients with COVID-19 pneumonia. Methods and Materials: We enrolled patients with moderate COVID-19 pneumonia who were older than 60 years. Participants were treated with LD-WLI in a single fraction of 0.5 or 1.0Gy along with the national protocol of COVID-19. The primary endpoints were improvement of SpO₂, the number of hospital/ICU stay days, and the number of intubations after RT and the secondary endpoints were alterations of the c-reactive peptide, interleukin-6, ferritin, procalcitonin, and D-dimer. The response rate (RR) was defined as a rise in SpO₂ upon RT with rising or constant trend in the next two days, and clinical recovery (CR) included patients who were discharged from the hospital or acquired SpO₂ ≥93% on room air. Results: Between 21 May 2020 and 2 July 2020, ten patients were enrolled. The median age was 75 years, 80% were male, and 80% had comorbidities. The first five patients received a single 0.5Gy-WLI, and others received 1.0Gy. Patients were followed for 2-14 days (median 5.5 days). Following one day, nine patients experienced an improvement in SpO₂. Five patients were discharged (median 6^th day, range 2^nd-14^th day), and four patients died (median 7^th day, range 3^rd-10^th day). Overall, the RR and CR were 60.0% and 55.5%, respectively. The RR and CR rates of 0.5- and 1.0Gy group were 80% vs 40% and 75% vs 40%, respectively. No acute radiation-induced toxicity was recorded. Conclusions: LD-WLI with a single 0.5Gy fraction seems to be a more appropriate dose to warrant further evaluation in a large-scale, randomized trial.