This research discusses Merger and Acquisition (M&A) decisions’ impact on the Indonesian Companies’ Operating Performance (OP). It measured the impact of the company’s OP before and after the M&A’s decisions have been taken by using operational performance data of the listed Indonesian company. Various results have been shown by the previous research in determining the impact of the M&A’s decisions on the company’s performances. Ramachandran Azhagaiah and Thangavelu Sathiskhumar (2014) discover that M&As strategy has the positive and significant impact on the Indian companies. In contrary, there are some studies that result in insignificant impacts in M&A. Scherer (1988) argues that most of the companies which made the M&A strategy have experienced profitability stagnancy in the long term. These research gaps become the foundation to address the issue in this research. This research was conducted for the Indonesian companies which took the M&A’s in 2011 by using the panel data including time series and cross-sectional data between 2006-2016. The time frame (2006-2016) was chosen to synchronize with this research’s goal, that is examining the OP within five years both before (2006-2010) and after (2012-2016) the M&A’s choices (2011). In addition, this research uses the multiple regression method with dependent variable which represent the company’s performance in related to OP namely, Profit Margin (PM), Gross Earnings (GE), Liquidity (L), Financial Risk (FR), Turnover (T), Growth (G), and Financial Leverage (FL). Using this method, the hypothesis of this research, whether there were significant differences in the company’s OP before and after conducting the M&As can be answered. Theresearch shows that the M&A strategy has significant effect on OP of the acquiring firms in Indonesia after M&A over the research period