This article examines how investor sentiment affects stock returns on Vietnam's stock market. Investor sentiment index is measured by a relative strength index (RSI) of 57 companies listed on the Ho Chi Minh Stock Exchange from January 1, 2015 to July 31, 2020. Control variables include investors' stock trading behavior, firm size, and cash flow per share. Using Fama-MacBeth regression estimation and general least square estimation (GSL) on a daily basis, both methods find the sentiment of high investors producing higher stock returns, on the contrary, the sentiment of low investors erodes stock returns. Different from the results of Brown and Cliff (2004) [Brown, G. W., & Cliff, M. T. (2004). Investor sentiment and the near-term stock market. Journal of empirical finance, 11(1), 1-27], the article found that the investor sentiment factor plays the most important role in explaining the return of the stock market compared to the rest of the factors.

This article examines 57 companies listed on the Ho Chi Minh City Stock Exchange from January 1, 2015 to March 31, 2020, to check whether or not investors' sentiment impacts the return of these stocks. Investor's daily sentiment towards each stock was measured by the psychological line index. The results of the study using both regression methods, Fama-MacBeth and General Least Squares, show that the influence of investor sentiment to return stocks in the stock market was significant and could not be ignored. Besides, the psychological factor of investors, their trading behavior causing stock supply-demand imbalance were all important factors affecting stock return. The study also found that the effect of firm size on stock returns was more pronounced when both investor sentiment and behavior were used in research models. The finding from this study suggests that individual investors who trade stocks every day can use psychological line index, which is one of the groups of decision-making indicators.