The present study aims at investigating the learning strategies on which Thai EFL learners rely in learning English relative clauses (ERCs). Not only do these strategies facilitate their ERC acquisition, but they are also found to lead them to certain kinds of problems. Such problems related to the learning strategies are first language transfer, transfer of training, avoidance, and overgeneralization. Thai learners seem to transfer a lack of some relative clause types in their native language to learning L2 ERCs. They also avoid the RC constructions, e.g. the object-of-preposition relative and the genitive relative, as well as the relativizers "whose" and "whom", which they probably consider too complex or unfamiliar. In addition, the previous ERC instruction apparently has a negative effect on their subsequent ERC learning, making the learners overproduce the relative marker "that." Furthermore, they even extend the use of "that" to non-restrictive RCs, which is grammatically incorrect in English. (Contains 5 tables and 4 footnotes.)