To build up the empirical description of ELF’s morphosyntactic system, this study examines Thai ELF users’ characteristics of relative clauses and analyses the causes underlying their use of the construction. Data were taken from the writing of 116 advanced and upper-intermediate English majors at a university in Bangkok. The results indicated that Thai ELF users’ relative clauses exhibit preferences for unmarked and explicit structures. Both linguistic and functional causes are responsible for their production of the construction: they form relative clauses full of basic and transparent structures in order to ensure simple and successful communication. The overall results reveal the nature of elf communication. Although Thai ELF users’ relative clauses are associated with some specific patterns, their use of this grammatical construction is governed by general linguistic processes which have been found to operate in the interactions of many groups of elf speakers, especially at the phonological and pragmatic levels.