Objectives: To assess peri-operative complications with the Modified Clavien Classification System (MCCS), evaluate the feasibility and safety, and explore predictive indexes for peri-operative complications in transurethral en bloc resection of bladder tumors (ERBT) procedure for non-muscle-invasive bladder cancer (NMIBC). Materials and methods: We retrospectively collected data of 162 consecutive patients who received ERBT for NMIBC from July 2014 to June 2016. All peri-operative complications occurred within 30 days were graded according to MCCS. Logistic regression analysis was used to investigate the impact of each of the factors on the incidence of perioperative complications. Results: Thirty nine complications were seen in 33 (20.3%) patients. The most common complication was bleeding related in 21 patients (63.64%). Four (2.47%) patients experienced extra-peritoneal bladder perforation and 3 of them received conservative treatment. Grade I and II complications constituted the majority (90.91%) while grade III was quite uncommon (only 9.09%). Multivariate analysis showed that the operative time, ASA score (3) and multiple resection sites were independent predictors of peri-operative complications. Conclusions: MCCS can be considered a practical and standardized tool in evaluating peri-operative complications in patients undergoing ERBT. Our results also confirmed that ERBT is a feasible and safe procedure for NIBC with a low complications rate regardless of the energy source. Multiple resection sites, ASA score >= 3 and longer operative time can be used as independent predictors for peri-operative complications.