The aim of this paper is to investigate some semantic interlingual errors in the writing performance of Libyan English as Foreign Language Learners (LEFLLS). The study seeks to analyze these errorsthat appear in the learners’ written production. The data was gathered from 25 essays written by 25 Libyan postgraduates majoring in different fields. It was collected and organized according to the errors committed. Errors taxonomy adapted from James’(1998) and Al-Shormani and Al-Sohbani’s (2012) was used in identifying and analyzing these errors. Semantic errors were classified into five categories namely formal mis-selection, formal misformation, lexical choice, collocation and lexico-grammatical choice. The semantic errors identified were 346. These errors were classified into four categories namely, formal misformation (46.7%), distortion due to spelling (32.4%), lexical choice (40.6%) and lexicogrammatical choice (2.4%). The results show that direct translation from the first language (L1), assumed synonym and misselection of letters sub-categories score the highest number of the errors, ie 14.2%, 13.08% and 12.08% respectively. While the sub-category, both collocations incorrect error was the lowest (0.52%). The other errors take the form of paraphrase (11.5%) and Idiomacity (8.5%). Two main sources have been found to be the cause behind these errors namely, L1 influence and insufficient knowledge about the second language (L2). Moreover, cultural differences between L1 and L2 had its impact in the written products of Libyan students.