Colloquialism is a remarkable language level that reflects the social and cultural influences of certain people. This study is an attempt to figure out the problems of translating Egyptian colloquial terms in Naguib Mahfouz's novel "Children of the alley" into English. It is challenging for a translator to correctly convey the meanings of these lexical items while preserving their content and effect. The study aims to identify the appropriate translations and the best strategy to adopt in addition to clarifying whether the translator maintains the same level of formality or switches to another one to preserve the meaning. To realize the previously-mentioned aims, the study hypothesizes that translators face difficulties when translating colloquial terms since they are linked to socio-cultural ties and reflect the authors unique mindset. Also, there is no obvious translation equivalent for several common colloquial terms. Thats why translators may switch to the formal level of language when dealing with colloquial expressions. Vinay and Darbelnets model (1958/1995) will be applied to colloquial items selected from Naguib Mahfouzs novel "Children of the alley" in terms of analyzing and evaluating the two translations submitted by the two translators: Stewarts (1981) and Theroux (1999). The study finds out that the absence of equivalence in most cases plays a vital role in making the process of translation more problematic. Therefore, a translator is preferred to switch to formal TL terms over informal ones, despite the fact that he/she does not maintain the authors attitude behind using such a term.