This study was designed to investigate the translation of laws from Thai into English with a focus on the use of modal 'shall' in the translations. As the trend of plain legal English continues to rise in the contemporary legal environment, some legal practitioners -- lawyers, judges and legal drafters -- have perceived that the use of the modal verb 'shall' (or the modal 'shall') may seem inappropriate and even to some extent, unclear in legal texts. Observing this phenomenon ultimately led to the formation of this study, and in following this line of argument, the research focuses on the extent to which the modal 'shall' is used by translators in the English translated versions of Thai laws, thus raising the research question: in which circumstances and to what extent do translators use the modal 'shall' in law translations from Thai into English? The study mainly adopted the concept of translation equivalence, focusing on the source-text and target-text clauses and sentences of the legal language, as an overall theoretical framework. The concept helps to indicate the extent to which the modal 'shall' is used in the English translations. As for the data, a total of 17 Thai laws (published by the Office of Council of the State of Thailand) and their corresponding translations were collected and used. The clauses where the modal 'shall' is used were then sorted out to create a parallel corpus for the analysis. The findings show that the modal 'shall' represents multifaceted denotative meanings of the verb in the Thai-source texts -- referring to 1) obligation; 2) declaration of facts; and 3) futurity in Thai laws -- which interestingly, deviate from the plain legal English movement. From the perspective of plain legal English, the modal 'shall' is only used to serve the language of obligation, often referring to a situation where 'someone has a duty to do something'