This paper shows that cognitive synonyms feature a serious problem in Arabic English translation. The study explores the translation of a number of cognitive synonymous lexical items in their original context of use. These synonyms were taken from Mahfouz’s two famous novels: “Al-Liss wal-Kilab” and “Ziqaq Al-Midaq”. Some other examples were also drawn from the Holy Quran. Thirty M.A students of Translation at An-Najah National University were the subjects of this study. They were requested to render these synonyms in their original contexts. The study explains that the translation of cognitive synonyms has been frequently done formally, functionally or ideationally depending on two significant factors: the text-type in which they are used and the purpose of using them in a particular context. The study, however, argues that in religious and literary texts where synonyms are usually used to convey certain implicated meanings and where we seek to have the same effect on the Target Language (TL) receiver as that of the original on the Source Language (SL) receiver, the use of formal equivalence is ruled out and hence only functional and/or ideational equivalence should be called for. Keywords: Translation ,Cognitive Synonyms ,Arabic Language, translation theory.