The present study investigates how Korean learners of English perform a speech act, in comparison with Korean native and English native speakers. With a focus on the speech act behavior of ‘gratitude’, this study aims to reveal the cross-cultural differences between the Korean and the English language groups, and to find what should be included in L2 sociolinguistic competence. The data have been obtained from 43 Americans and 91 Koreans assigned to three groups: English native speakers, Korean learners of English, and Korean native speakers. A discourse completion test and a subsequent interview examined the participants’ perception of the social factor of power relationship and further their choice of response strategies according to the power relationship. The statistical procedure revealed that by different power relationships, they used significantly different gratitude strategies as well as politeness markers. Korean learners of English heavily depended on the strategies of negative politeness (Brown & Levinson, 1978. 1987), and the pattern of usages was similar to the Korean language group. Korean EFL textbooks and classrooms, therefore, need to include not only more diverse contexts in which ‘gratitude’ is necessary, but also the ways in which the English speech act is performed in a culturally appropriate manner.