© 2020 JLLS and the Authors.In today's world, educational contexts are getting increasingly multicultural. Although EFL (English as a Foreign Language) classes in Turkey were mostly composed of Turkish students a decade ago, today students from any part of the world are brought together within the frame of various student exchange programs. In Turkish EFL contexts, students coming from Middle Eastern countries make up a huge part of classes (Özer, 2016). The objective of the current study is to examine L1 negative interference errors of Turkish and Arabic EFL learners. Accordingly, 30 B1 (Intermediate) EFL students' written assignments were examined and divided into categories. Besides, this study makes a comparative analysis of Turkish and Arabic EFL learners' grammatical and lexical errors within the frame of L1 negative transfer. When the results of the Mann-Whitney U test are analyzed, it is understood that Turkish and Arabic students' grammatical interference exam scores differ statistically (p <0.05). When the total amount of errors is analyzed, Turkish students' grammar errors (n=164) stem from, majority of which are articles, preposition and tense errors, respectively as 18.82%, 17.65% and 11.76 whilst Arabic students make a huge part of their errors (n=352) on capitalization, punctuation and tenses/articles, respectively as 16.19%, 15.06%, and 12.78%. In the frame of the lexical category, Turkish students performed 48 lexical errors, the majority of which are spelling and collocation errors. On the other hand, Arabic students committed 96 lexical errors in total, the majority of which are spelling errors (60.4%) while 15.5% of the errors are wrong word choice. In general, lexical errors were fewer than grammatical errors.