The present study aimed to investigate Mazandarani-Persian Bilinguals' L3 learning of simple present tense to see whether their L1, L2, or dominant language of communication affect their L3 learning. To this end, 90 male and female students, with the mean age of 14.5 were selected from among the elementary L3 learners of English. The participants were assigned into three groups of L1 Mazandarani/L2 Persian/ L3 learners of English. The first group had Mazandarani as the dominant language of communication while the second and the third group had Persian as the dominant language of communication. The Grammaticality Judgment Task and Oral Translation Task were used to check the participants' production and comprehension of the target structure. The results of the Kruskal Wallis and the Mann Whitney U tests showed that the dominant language of communication, irrespective of its status as the first or second language, was the primary source of cross-linguistic influence in learning simple present tense at the initial stages of learning this tense.
