This study aims to examine the relationship of the four factors that increase the protection of minority shareholder investment. The factors are non-controlling shareholders, corporate governance, free cash flow, and shareholder wealth. The data for this study is obtained from the 2017 annual reports of 136 Thai public companies listed in the Market of Alternative Investment of Thailand (MAI). The analysis uses a multiple regression model to determine which factors encourage and which inhibit the protection of minority shareholder investment. The study tests four hypotheses. The results rejected H1 because non-controlling shareholders have negatively correlated with minority shareholder investment protection (beta -0.155 and p-value 0.050). The results accepted H2, H3 and H4 as follows. H2: corporate governance has positively correlated with minority shareholder investment protection (beta 0.17 and p-value 0.031). H3: free cash flow has positively correlated with minority shareholder investment protection (beta 0.214 and p-value 0.007). H4: shareholder wealth has positively correlated with minority shareholder investment protection (beta 0.318 and p-value 0.000). The major findings suggest strong minority shareholder investment protection was enhanced by increasing corporate governance, free cash flow and shareholder wealth. The protection of minority shareholder investment needs to reduce non-controlling shareholding pattern.