This study aims to explore how the local environment influences Corporate Social Responsibility (CSR) of Korean Multinational Corporations (Korean MNCs) operating in Southeast Asia. In-depth interviews were conducted with executives of five Korean MNCs operating in Thailand and Vietnam. All interviewed companies had the character of weak CSR structure and unsystematic CSR approach in common. Within these organizational limitations, different domestic environment between the two countries affected their CSR development. The interviewed companies in Vietnam performed strategic CSR, which responded to the social issues and the requirement of host country, under the greater pressure on CSR from the Vietnamese society. There were various external forces in CSR decision-making channel, and they recognized the local government as the object of solidarity and cooperation in CSR. In Thailand, the interviewed companies accepted CSR as a part of the culture, which is on voluntary basis and donation-centered, or did not receive any influence from the domestic environment. As a result, their CSR performance have been mostly ‘responsive CSR’, which means one-off event or short-term philanthropy. Consequently, the result revealed that the different domestic factors, such as the government’s policy, social pressure and culture regarding CSR, largely affect CSR strategy and activities of Korean MNCs in Thailand and Vietnam.