Purpose - International diversification, a critical issue in research and practice, undoubtedly represents a growth strategy with great potential impact on firm performance. Although the literature has widely examined the relationship between international diversification and firm performance in international business and strategic management research, it remains unconfirmed. This study, therefore, aims to clarify the relationship as it relates to information service firms. Research design, data and methodology - To examine the international diversification-performance relationship, this study uses a sample of major Korean service firms spread over four industries spanning a period of five years between 2005 and 2009. In this study. generalized least square was used to analyze the relationship between international diversification and performance of service firms. Result - The result showed the U-shaped curvilinear form between international diversification and performance in service firms. Especially, IT service firms may differ from other service firms that the negative effect of internationality may have weaker than other types of service firm. Conclusion - Based on the empirical results, the U-shape relationship between international diversification and performance is confirmed in Korea service firms. Furthermore, information service firm may reduce the initial internationalization cost by transferring codified and digitized information using information and communication technology, so that gain benefit from early international expansion.