Flies of the order Diptera are of the most important insects and of wide distribution in all parts of the world, except in the Antarctic Circle. Some of them are pests of highly economic important plants; many are vectors of dangerous diseases for man and animal and cause threat in various parts of the world and exert effective serious problems (Borror and Triplehorn, 1976; Hendel, 1936-37; Lindner and Erwin, 1928; Oldroyd, 1949 in addition to Yeast and Wiegmann, 2005).             The recent world catalogues of the order lists over 100 valid families and approximately 240.000 species (James, 2008), of them 65 families, and 1339 species (Steyskal, 1967) are recorded in Egypt. Due to the lack of illustrated taxonomic keys for the representative families of the order in Egypt, the present work was planned to construct a new key to facilitate the recognition and separation of the families and for the benefit of entomologists interested in the dipterous insect fauna of Egypt. The present key cover the various dipterous keys in the literature, text books, taxonomic catalogues and the work of many authors, (Nagatomi, 1991; Wood, 1989; Yeates and Wiegmann, 1999; Thompson, 2005; Evenhuis, 1994; Hennig, 1973; Wood and Borkent, 1989; Wood, 1991; Kristensen, 1991; Griffiths, 1996; Kukalova-Peck, 1991; Merritt et al., 2003; and Kitching et al., 2005; McAlpine, 1989). Who gave the main classifications of the higher categories of the order, in other hand many literature that deal with the main morphological characters are used (McAlpine and Wood, 1983; McAlpine and wood, 1981&1987; Wiegmann, & Thompson, 1993; Verral, 1909 and wood, 1991).             Many particular references that deal with separated suborders and there sections by using a morphological characters or by phylogeny and classification of the suborders are recognized for the purpose of the key, (Bickel, 1982; Evenhuis, 1995; Hendel, 1928; Papavero, 2003; Wiegmann and Kishino, 2003; Wood, 1991 and yeast, 2007) to explained and provide a true variation in many families of the order.      The present work is considered also as a contribution and updating to the work of (Steyskal, 1967), who provided a checklist of order Diptera in Egypt including families and species together with their synonyms and representative material in various insect collections and other taxonomic notes and a key to the families. It is hoped that this work will serve and provide a stimulus for further taxonomic and zoogeographic work.