This study investigates the impact of L1 (Vietnamese) on L2 (English) with a specific reference to rhythm. The transfer of L1 rhythm to the L2 rhythm is considered as a potential factor in reducing EFL learner's intelligibility. The findings of this quantitative research show that Vietnamese adult EFL learners' oral output is significantly unintelligible due to the application the mid-level tone of their first language, part of Vietnamese rhythm to produce English rhythm in a pronunciation test, making their utterance monotonous. This paper aims at presenting findings gained from an examination into how the Vietnamese rhythm, the mid-level tone, affects 50 Vietnamese adult EFL learners' intelligibility in a pronunciation test assessed by ten judges