Abstract: Objective To investigate the molecular mechanism of long non-coding RNA (lncRNA-ATB) activated by transforming growth factor to promote invasion and metastasis of breast cancer cells and resistance to trastuzumab. Methods Fifteen patients with breast cancer who were resistant to trastuzumab and underwent surgical resection of the tumor were selected to detect the mRNA levels of lncrNA-ATB in the cancer tissues and adjacent tissues by fluorescence quantitative reverse transcription polymerase chain reaction (RT-PCR). Patients were followed up for 36 months after surgery, and the expression of lncRNA-ATB was plotted with the progression-free survival time (PFS) and total survival time (OS) by Kaplan-Meier method. Normal breast cells MCF-10A and breast cancer cells MCF-7, MDA-MB-435S, T47D and ZR-75-30 were selected to detect the mRNA level of lncRNA-ATB by RT-PCR. After lncRNA-ATB was silenced by small interfering RNA (siRNA), changes in the sensitivity of cells to trastuzumab were detected by CCK8 (cell counting kit-8). Flow cytometry was used to detect the apoptosis induced by trastuzumab. The effect of lncRNA-ATB on invasion and metastasis of breast cancer cells was detected by Transwell and wound healing test. Western blot was used to analyze the effects of lncRNA-ATB on multidrug resistant protein 1 (MDR1) and epithelial-mesenchymal transformation (EMT) indicators (E-cadherin and vimentin). Results The mRNA level of lncRNA-ATB in cancer tissues of patients with breast cancer was significantly higher than that in the adjacent tissues (P<0.01). The high expression of lncRNA-ATB indicated the shorter survival time of patients (P<0.05). lncRNA-ATB knock out significantly increased the cell sensitivity to trastuzumab, the half maximal inhibitory concentration(IC₅₀)decreased significantly (P<0.05), and the level of apoptosis induced by trastuzumab was significantly increased (P<0.05). In addition, the ability of cells to invade and migrate was significantly reduced. Western blot results showed that the expression level of drug-resistant protein MDR1 decreased significantly (P<0.05), and the EMT process was significantly reversed. Conclusions lncRNA-ATB plays an important role in the development of trastuzumab resistance and invasive metastasis in breast cancer, and lncRNA-ATB may be a potential prognostic indicator and a potential therapeutic target for clinical breast cancer patients.