Background: In platinum eligible patients with advanced urothelial carcinoma (UC), there are two treatment options if they did not show progression after 4 cycles of first line chemotherapy. One is switching to avelumab maintenance therapy and the other is pembrolizumab after the disease progression on first line chemotherapy. However, there is no data witch treatment strategy demonstrate better survival. This study aimed to compare the efficacy between avelumab and pembrolizumab after first line chemotherapy without disease progression. Methods: We retrospectively reviewed patients with advanced UC who received avelumab or pembrolizumab as subsequent therapy following first line platinum-based chemotherapy between March 2018 and Jun 2023. In patients who receive pembrolizumab, only patients who did not show progression after 4 cycles of first line chemotherapy were included in the study cohort. The oncological outcomes were compared between the patients who received avelumab and pembrolizumab. Results: Thirty-three and 100 patients received avelumab and pembrolizumab during study periods, respectively. Among 100 patients who received pembrolizumab, 14 patients did not show progression during 4 cycles of first line chemotherapy. Median 4 and 6 cycles of first line chemotherapy were performed in patient treated with avelumab and pembrolizumab, respectively. All patients treated with pembrolizumab showed disease progression after 1st line chemotherapy. With the median follow-up of 16.7 months, progression was observed in 19 (57%) and 12(86%) patients in avelumab and pembrolizumab group, respectively. Among the patients with progression, all patients had treated with avelumab received subsequent treatment mainly enfortumab vedotin. However, only 2 patients (17%) had treated with pembrolizumab received subsequent treatment and the other patients were not candidate of additional treatment due to poor PS or cachexia. Although median progression free survival from starting first line chemotherapy was shorter in patients with avelumab compared to pembrolizumab (9.7 vs. 19.9 months, P=0.06), median OS was longer in patients with avelumab (40.9 vs. 24.9 months, P=0.34). Conclusions: In patients who did not show progression after 4 cycles of first line chemotherapy, avelumab maintenance therapy might be a better treatment option with longer overall survival compared to pembrolizumab.