Objective:To evaluate the incidence of arrhythmias and electrocardiographic (ECG) characteristics in cancer patients treated with immune checkpoint inhibitors (ICIs).Methods:This was a cohort study conducted in the Fourth Hospital of Hebei Medical University. Cancer patients initiating ICIs treatments from November 2020 to September 2022 were included in this study. Baseline 12-leads ECG before ICIs initiation and post-treatment ECG were analyzed. An abnormal ECG was defined as the presence of any of the following changes: sinus arrhythmias, atrial fibrillation, atrial flutter, paroxysmal supraventricular tachycardia, ventricular tachycardia, premature contractions, conduction disorder, and ST-T changes.Results:A total of 87 patients were enrolled, aged 63 (57, 68) years, with 66 (75.9%) males. And 44.8% (39/87) of patients presented with at least one confirmed cardiovascular disease or cardiovascular risk factor at baseline. The incidence of abnormal ECG increased from 31.0% (27/87) at baseline to 65.5% (57/87) after receiving (5.0±2.7) cycles of ICIs treatment ( P<0.001). The incidence of sinus arrhythmias was significantly increased after ICIs treatment (23.0% (20/87) vs. 9.2% (8/87), P=0.023), of which only the incidence of sinus tachycardia was significantly increased (11.5% (10/87) vs. 2.3% (2/87), P=0.039). There was also a significantly increased incidence of ST-T changes after ICIs treatment (31.0% (27/87) vs. 17.2% (15/87), P=0.012), which mainly attributed to the T wave changes (29.9% (26/87) vs. 13.8% (12/87), P=0.001). The incidence of premature contractions was also significantly increased after ICIs treatment (9.2% (8/87) vs. 0, P=0.008). Additionally, compared with baseline, the P wave axis was significantly increased after ICIs treatment ((56.94±21.01)° vs. (52.00±22.69)°, P=0.043). After ICIs treatment, the heart rate was significantly increased ((79.07±15.37) beats/min vs. (75.64±13.37) beats/min, P=0.029). Sokolow-Lyon index ((2.21±0.81)mV vs. (2.33±0.75)mV, P=0.138), QTc interval ((431.44±36.04)ms vs. (428.00±30.05)ms, P=0.415) all showed signs of change after treatment, but did not reach the traditional significant level. Conclusions:The incidence of abnormal ECG is significantly increased after ICIs treatment, especially for sinus tachycardia, premature contractions and T wave changes; the P wave axis and heart rate is also significantly increased after treatment. It is important to perform regular ECG monitoring in patients receiving ICIs treatment.