Background: Trastuzumab emtansine (T-DM1) was established as a standard second line treatment option for metastatic HER2 positive breast cancer (mBC). However, the EMILIA study did not include disease progression patients who received pertuzumab plus trastuzumab previously. After CLEOPATRA study, the dual blockade treatment was indicated for advanced breast cancer as first line treatment. In this study, we attempted to build the T-DM1 predictive model and to evaluate the association between dual blockade and T-DM1 for metastatic HER2-positive breast cancer. Methods: This retrospective cohort study was conducted in Taiwan. Consecutive patients with metastatic HER2-positive breast cancer who had received T-DM1 between January 2013 and December 2022 were enrolled. Those patients were divided to two groups based on strategy of trastuzumab only or dual-blockade as prior therapy. Demographic, clinical, and pre- and post-T-DM1 treatment characteristics were described. The swimmer plots were used to estimate time to treatment discontinuation (TTD) and overall survival (OS). Results: Of 32 patients treated with T-DM1, 12 (38%) had prior treatment with pertuzumab plus trastuzumab. The median age was 60 years. Most patients had visceral disease and 59% had two or more prior treatments for mBC before T-DM1 (range 1–8 lines). Most patients (63%) stopped T-DM1 due to disease progressed and followed by financial problem (22%). In dual-blockade group, median TTD with T-DM1 was 4.5 months; median OS from diagnosis day was 68.2 months (8 patients still alive in the end of study). In trastuzumab only group, median TTD with T-DM1 was 3.8 months; median OS from diagnosis day was 73.2 months (5 patients still alive in the end of study). In our swimmer plots we found a trend that the shorter interval between trastuzumab +/- pertuzumab and T-DM1 the longer TTD with T-DM1. Conclusions: Patients treated with T-DM1 in this study appeared to have more advanced disease than patients in clinical trials and were treated in later lines of therapy. We should prescribe T-DM1 once trastuzumab with or without pertuzumab failure for the longer TTD. The small patient number was a major limitation in our study. A multi-center analysis is needed and it will help us for treatment suggestion under the Taiwan’s National Health Insurance clinical practice.