In this thesis, we discuss the possible diversification benefits offered by commodity futures, especially in the extreme equity market conditions. We see that adding commodity investments into portfolios could improve their performance with better diversification efficiency. However, with correlation estimation by methods developed by Longin and Solnik(2001) and Ang and Chen(2002), we see correlations between equity and commodity investments increase while they are on downside moves . The results suggest that the diversification benefits offered by commodity investment may change in different market conditions. For further examination, we divide our sample in groups ranked by the home market (U.S. equity investments) returns to see if the commodity-equity portfolio could still perform better over all-equity portfolio in different times. The statistical test shows that the commodity-equity portfolios still perform better than all-equity portfolios. We conclude that commodity investments could make portfolios better-diversified, no matter how the market conditions are.