OBJECTIVES: Trastuzumab, a monoclonal antibody, has been widely used in treatment of HER2-positive breast cancer because of its proved effectiveness and safety. However because of the high price, the cost-effectiveness of trastuzumab should be evaluated especially in such low-income country as Vietnam. This is also the aim of this study METHODS: A Markov model has been constructed with 5 health states (disease-free survival, local recurrence,  regional recurrence, metastatic, death) with 1-year cycle length and lifetime horizon. The transition rates between states have been retrieved from relevant epidemiological studies, clinical trials and experts’ opinions. A population of 1000 50-year-old women with average weight of 60 kg has been included in model. Lists of medical services and drugs were derived from NCCN guideline 2014. The prices of drugs and medical services have been averaged from the price-lists in 2013 of some major hospitals in Vietnam. Both QALYs and cost were discounted at 3%. Probabilistic sensitivity analysis was also conducted. RESULTS: