Scholarly interest in legislative translation has grown substantially over recent decades, with corpus-based approaches contributing to our understanding of the relationship between translated legislation and source texts, on the one hand, and translated and non-translated legislative texts in the target language, on the other. To date, however, most studies have been conducted on European languages. This study is part of a first attempt to use corpus techniques to explore legislative translation from English into Thai. Drawing on a purpose-built, 400,000-word, parallel corpus of international treaties translated from English into Thai, and a one million-word monolingual corpus of legislative texts originally written in Thai, we investigate how instances of deontic modality are translated into Thai. We analyse the modal strength of translations and conduct our inter-linguistic and intra-linguistic comparisons in the light of Biel’s (2014) concepts of equivalence and textual fit.