In surveys of third language acquisition (TLA) research, mixed results demonstrate that there is no consensus among researchers regarding the advantages and/or disadvantages of bilinguality on TLA. The main concern of the present study was, thus, to probe the probable differences between Persian monolingual and Azeri-Persian bilingual learners of English regarding their syntactic knowledge. It was an attempt to investigate whether they differ significantly in learning relative clauses. To carry out this study, a total of 200 female high school students studying at second grade were randomly selected from two educational districts of Tabriz and Shiraz in Iran. The participants were homogeneous in terms of English proficiency, sex, and age. They attended public high schools they were taught the same materials and had the same number of hours of instruction. A general proficiency test, a language history questionnaire, and two syntactic structure tasks were administered to both groups. Statistical analyses including t-tests and descriptive statistics revealed that monolinguals and bilinguals differ in the comprehension and production of English L3 relative clauses.