In this paper, we analyze the effect of M&A investment strategy on company performance, considering the level of diversification. Firstly, we built datasets that control environmental factors by using propensity score matching. Then we analyzed the matched data by multivariable regression analysis. The ratio of M&A investment to internal investment has a significant positive effect on Return on Invested Capital, or ROIC, when companies are highly diversified. When the level of diversification is low, the effect becomes negative. Therefore, M&A strategy is considered to be an effective diversification mode for highly diversified companies.