The current study aimed to investigate the production of L2 English ‘wish-clauses’ by L1 Thai learners, based on the Interlanguage Hypothesis (Corder, 1981; Selinker, 1972, 1992). It was hypothesized that L1 Thai learners showed systematicity in their interlanguage in the use of ‘wish-clauses’ and that their IL was shaped by language transfer and transfer of training, which are among the psychological processes of IL construction. English ‘wish-clauses’ examined in the study were three types of hypothetical or counterfactual wish: wish about the present, wish about the past, and wish about the future. A Cloze Test and a Situation Task were administered to 30 L1 Thai undergraduate students, divided equally into two groups: the intermediate group and the advanced group. The results conformed to the hypotheses in that Thai learners of both proficiency levels seemed to exhibit similar systematicity in the production of all types of English ‘wish-clauses’ in both tasks. Also, some systematicity was exhibited mainly in the intermediate group. The systematicity was assumed to be influenced by language transfer, in combination with transfer of training. The results of the study are expected to shed light on the characteristics of English IL of ‘wish-clauses’ among L1 Thai learners and the difficulties they face in the acquisition of English ‘wish-clauses’.