Objectives: Trastuzumab emtansine(T-DM1) is more effective in some patients. However, predictive factors are not clear for T-DM1 efficacy. We attempt to explore prognostic importance of albumin to alkaline phosphotase ratio (AAPR) and prognostic nutritional index (PNI) importance in patients who used T-DM1. Methods: The present retrospective study included the sixty-one human epidermal growh factor-2 (HER-2) positive metastatic breast cancer patients who used T-DM1. The median values were used to analyze AAPR and PNI threshold. PNI and AAPR were measured at baseline and after one cycle. Survival analysis was performed by using Kaplan-Meier method and compared with log rank test. Univariate and multivariate analysis were performed to determine independent prognostic predictor of progression-free survival (PFS) and overall survival (OS). Results: The optimal cutoff value for AAPR and PNI were 0.41 and 45.0, respectively. OS and PFS were obviously inferior in patients with AAPR<0.41 compared to patients with AAPR≥0.41 according to Kaplan-Meier curve (OS: p=0.028, PFS: p=0.047). Patients with PNI <45 prominently demonstrated poor OS and PFS than those with PNI≥45 according to Kaplan-Meier curve (OS: p=0.033, PFS: p=0.071). Conclusion: Pretreatment PNI is reliable prognostic indicators in patients with mBC treated by T-DM1.