Mergers and acquisitions are one of the most researched topics in the financial literature and many research has been done. This thesis aims to analyse the impact of mergers and acquisitions on firms’ performance for the acquired firms and uses the difference-in-differences (DID) estimation method with variation in treatment timing to achieve the objective. The dataused in this thesis are collected from database of Institute for Mergers, Acquisitions and Alliances (IMAA) for 25 companies that merged between 2005 and 2019 and from the database of Indonesia Stock Exchange for 25 firms that did not undertake the mergers and acquisitions in the same period. The findings of this study indicate that there are statistically not significant effects on Return on Assets (ROA) and Return on Equity (ROE). This suggests that the mergers and acquisitions do not affect the company's financial performance in terms of profitability.