The Vietnamese government introduced a change in the minimum tick size for stock trading on 12 September 2016 to improve market quality and reduce trade execution costs. The intended effects of this policy have not been widely investigated in an emerging market such as Vietnam. We use data on trade and quote intraday of all stocks listed on the Ho Chi Minh Stock Exchange for the periods before and after the event, with a one-week break from 12/9/2016 to 18/9/2016, for the market to adapt to the new tick size policy. Findings from this paper confirm that the trading cost is reduced following the change to the smallest tick size. However, this is different for large trades executed at the stock price associated with a larger tick size. Furthermore, the findings are robust with a different sample period. These findings imply that introducing a change in tick size in Vietnam in 2016 is desirable for improving market quality. However, the differentiation of these changes in different ranges of stock prices is not necessarily effective for improving market quality and reducing trade execution costs.