併購乃是企業發展與成長之重要策略工具之一，併購不僅可以擴大規模、降低成本，增加市場佔有率；併購亦可以增加產品的多元化，提升其獲利能力。為了解併購對於東亞國家之銀行業成本效率的影響，本研究首先使用Battese and Collei（1995）的隨機邊界成本函數法，針對日本、台灣、香港、南韓、新加坡等五國銀行業，同時估計隨機邊界成本模型及無效率模型，以了解主併銀行、被併銀行及合併後對銀行效率之影響。接著利用共同成本邊界模型，比較各國之成本績效，以及主併銀行合併前後三年的共同成本效率值的表現。本文的實證結果發現：除日本外，其餘四國均是主併銀行在進行合併時，可顯著提高成本效率。台灣、日本、新加坡之被併銀行與成本效率呈反向關係。在合併效益方面，五國的合併效益變數均與成本效率呈反向關係，意謂合併後3年內，因組織的調整等整合因素，的確會降低銀行的成本效率。在共同成本效率方面，台灣及日本一直是五國中表現最穩健的，而台灣的表現尤其突出。但無論在何階段，新加坡的共同成本效率都是最差的，推估與其過度熱衷於拓展海外版圖有關。進行併購行為之銀行，其成本效率最差時，大多發生於主併當年，進行併購後一年，則共同成本效率均會提高。

Merging and acquisition (M&A) is one of the most important strategies for the development and growth of enterprises. Not only M&A could increase scale, lower cost, expand market shares, but also improves the product diversity and profitability. In order to understand how M&A affects the banks performances among the five countries in East Asia, this study uses the stochastic frontier approach of Battese and Coelli (1995) to compare the different cost efficiencies among the banks of Taiwan, Japan, Hong Kong, Singapore, and South Korea. By simultaneously estimating the cost frontier model and cost inefficiency model, we find that, for most countries, the merging banks increase cost efficiency, while the merged banks decrease cost efficiency. By metafrontier approach of Battese et al. (2004), we find that, in term of both technical gap ratio and metafrontier cost efficiency, Taiwan's banks have the highest performance while Singapore Banks are the worst. Comparing the timing, the worst performance us ually happens on the merging year. However, the metafrontier cost efficiency would be increased one year after merged for all five counties.