Abstract: This study aims at gaining effects of a company’s internal factors such as the size of the company, financial risk, debt to equity ratio, dividend policy and its external factors such as interest rates, exchange rates, inflation and economic growth on investment opportunities as measured by Investment Opportunity Set (IOS) as well as the effect of investment opportunities on the firm value as measured by Tobin’s q in 5 ASEAN countries namely Indonesia, Malaysia, Philippines, Singapore and Thailand. The analysis of this study is a panel data of 100 huge capital companies engaged in the property since 2008-2013. The researcher of this study formulates the research problem as follows: what internal and external factors influence the company’s investment opportunities which are proxied through IOS either partially or simultaneously? And how does the investment opportunity proxied through the IOS affect the firm value as measured by Tobin’s q? This study used a simultaneous regression analysis for panel data and the Two Stage Least Squares (TSLS) methods to estimate parameters. The results show that there is a positive significant effect of Investment Opportunity Set (IOS) on the company value of the 5 ASEAN countries. The internal factors influence showed different results in each country. The macro-economic and environmental conditions affect the investment activities as described in the theory of Keynes and Minsky that stated that the micro and macro-sectors cannot be separated from one another.