Objective Early predictors of trastuzumab ⁃ related cardiotoxicity are urgently needed. There⁃ fore，we investigated the clinical value of the deceleration capacity of the heart rate（DC）in predicting trastuzumab⁃ related cardiotoxicity in the treatment of human epidermal growth factor receptor 2（HER2）⁃positive breast cancer. Methods Patients with HER2⁃positive breast cancer treated with trastuzumab at our hospital from June 2015 to June 2017 were retrospectively observed. DC was assessed before treatment ，and the left ventricular ejection fraction（LVEF）was regularly monitored for 2 years（before，during，and after treatment）to evaluate the develop⁃ ment of cardiotoxicity. Results Among 150 eligible patients with complete datasets，28（18.7%）developed cardiotoxicity. The age of patients with cardiotoxicity were older（P = 0.018），the anthracycline dose they received was higher（P < 0.001），and DC and baseline LVEF they had were lower（P < 0.001），compared with those in patients without cardiotoxicity. Logistic regression revealed that lower DC，lower baseline LVEF，and higher anthra⁃ cycline dose were independent risk factors of trastuzumab ⁃ related cardiotoxicity. Receiver operating characteristic curve analysis revealed a greater area under the curve for DC than for the baseline LVEF in predicting cardiotoxicity （0.875 vs. 0.768，P = 0.032）. Conclusion DC should be an early predictor of trastuzuma⁃related cardiotoxicity