The research investigated pragmatic transfer in responses to English tag questions by L1 Thai learners based on Interlanguage Pragmatics, specifically pragmatic transfer (Kasper & Blum-Kulka, 1993). The L1 Thai learners were categorized into two groups according to their English proficiency levels: advanced and intermediate. Oral and written discourse completion tasks (Blum-Kulka, 1982) were employed to elicit the participants' responses to English affirmative and negative tag questions in two modalities, speaking and writing. The major findings cast light on the L1 Thai learners' problems of responding to English negative tag questions, rather than positive ones, as a result of their strong reliance on the Thai pragmatic norm. The results also suggested that the responses to English negative tag questions by the intermediate group were less native-like than the advanced group's responses and manifested a higher degree of pragmatic transfer. Concerning pragmatic transfer in the two modalities, responses to English negative tag questions in writing showed a greater degree of pragmatic transfer than those in speaking. The results of the study are expected to elucidate the performance of the L1 Thai learners' responses to English tag questions in both modalities and their dependence on the Thai pragmatic norm in responding to English tag questions. [This study was presented virtually at the 32nd International Conference on Foreign/ Second Language Acquisition on May 20-22, 2021.]