The study investigates systematicity in English interlanguage of dependent prepositions among L1 The findings demonstrate that systematicity occurred in the learners' English usage of prepositions of all such types, possibly due to negative transfer from the learners' native language. Also, the L2 learners tended to exhibit such systematicity irrespective of their English proficiency level. It may be assumed that the cognitive aspect of L2 learners' working memory is involved in processing the usage of the four types of English dependent prepositions. The results of the study are expected to shed light on the problems of L2 English interlanguage of dependent prepositions among L1 Thai learners.