The poem mixes the West Saxon and Anglian dialects of Old English, though it predominantly uses West Saxon, as do other Old English poems copied at the time.
There is a wide array of linguistic forms in the Beowulf manuscript. It is this fact that leads some scholars to believe that Beowulf has endured a long and complicated transmission through all the main dialect areas. The poem retains a complicated mix of the following dialectical forms: Mercian, Northumbrian, Early West Saxon, Kentish and Late West Saxon.:20–21 There are in Beowulf more than 3100 distinct words, and almost 1300 occur exclusively, or almost exclusively, in this poem and in the other poetical texts. Considerably more than one-third of the total vocabulary is alien from ordinary prose use. There are, in round numbers, three hundred and sixty uncompounded verbs in Beowulf, and forty of them are poetical words in the sense that they are unrecorded or rare in the existing prose writings. One hundred and fifty more occur with the prefix ge- (reckoning a few found only in the past-participle), but of these one hundred occur also as simple verbs, and the prefix is employed to render a shade of meaning which was perfectly known and thoroughly familiar except in the latest Anglo-Saxon period. The nouns number sixteen hundred. Seven hundred of them, including those formed with prefixes, of which fifty (or considerably more than half) have ge-, are simple nouns, at the highest reckoning not more than one-quarter is absent in prose. That this is due in some degree to accident is clear from the character of the words, and from the fact that several reappear and are common after the Norman Conquest.