This excellent Gothic poem actually has a very regular rhyme scheme, that helps to contribute towards the unrelenting and inexorable pace of the poem that drives its speaker on in his tortured and frenzied thoughts as he tortures himself over the memory of his "lost Lenore."
When we work out the rhyme scheme of a poem, we look at a stanza and assign a letter to each separate rhyme that there is, repeating the letter is the same rhyme occurs. Thus, examining the first stanza, we assign the letter "A" to "weary," then "B" to "lore." "Tapping" represents another rhyme, so we give that line "C" and then finally we can see that "door," "door" and "more," the words that end lines 4, 5 and 6, match the rhyme of "lore," so we give these lines the letter B. Therefore the poem has a regular rhyme scheme that can be expressed in the following way: A B C B B B. This is continued throughout the poem.
There is also a great deal of internal rhyme in the poem. Internal rhyme was a common device among 19th-century poets, such as Poe, in which a word in the middle of a line rhymes with one at the end.
Consider the following example from the first four lines in the first stanza:
Once upon a midnight dreary, while I pondered, weak and weary,Over many a quaint and curious volume of forgotten lore—While I nodded, nearly napping, suddenly there came a tapping,As of some one gently rapping, rapping at my chamber door.
Ah, distinctly I remember it was in the bleak December;And each separate dying ember wrought its ghost upon the floor.Eagerly I wished the morrow;—vainly I had sought to borrowFrom my books surcease of sorrow—sorrow for the lost Lenore—
Then, upon the velvet sinking, I betook myself to linkingFancy unto fancy, thinking what this ominous bird of yore—What this grim, ungainly, ghastly, gaunt, and ominous bird of yoreMeant in croaking “Nevermore.”