From the impact of globalization in technology, there are a lot of new start up business especially in Indonesia because it enhances human’s creativity. This proven by the economic census that conducted by Badan Pusat Statistik (BPS) back in 2016 that found there are 26.71 million companies in Indonesia which increased by 17.51% compared with the findings of economic census in 2006. Meaning that there are 3.98 new companies in the last 10 years. To survive in the competition, companies are trying to expand their geographic reach and growth to survive in the market and there are many cases that the growth happens through merger and acquisition. The reasons why companies do merger and acquisition is to realize the synergy. However, not all merger and acquisition are success. Financial performance, market performance, stock return, and stock risk can be used as the measurement to assess whether merger and acquisition give any difference in the companies. With those measurements as the variable, this study uses Paired Sample T-Test, Wilcoxon’s Signed Rank Test, and F-Test for two different variances for the hypothesis testing with the period of 3 and 1 years before and after the event and 5 days before and after the event. From the observation, this study finds there is a difference in the companies ROE, ROA, total asset turnover in 3 years and 1 year before and after the event, price/book value 1 year before and after the event, and stock risk 1 year before and after the event. From those findings, this study suggests companies that are planning to do merger and acquisition to consider the strategy in larger base of capital and fix asset before conducting merger and acquisition also consider about the debt to equity and debt to asset ratio that keep on increasing with the total asset turnover that keep on decreasing. While for the investor, price/earnings and price/book value can be the indicator for the companies’ future growth and consideration in investing and also for those investors who are not a risk taker, better to wait until the event realized because the stock is tend to riskier before the event. Keywords: Merger, Acquisition, Synergy, Financial Performance, Market Performance, Stock Return, Stock Risk.