Frankenstein also conforms to the horror genre in literature because of its style. Mary Wollstonecraft Shelley's novel is an example of the Gothic genre in literature: a genre that contains certain elements of plot, character and style which conform to established standards. This genre contains elements of both horror and romance, and usually there is some kind of romantic intrigue at the center of the story. The writing style in Gothic works is usually very descriptive, dramatic and concerned with establishing an enticing and mysterious atmosphere within which the story can unfold. By creating a mysterious environment steeped with realistic sensory details, the often-fantastic elements of Gothic stories become more plausible.
Well, Frankenstein does belong to the Gothic genre, but I think its generic status is not very simple. It is extremely self-reflexive in its use of the Horror-genre and it can also be read as an appropriation of the Gothic genre to raise some still other issues and themes such as the theological paradox of creation, the relation between god and man, the debates about the Divine figuration of the writer in the Romantic aesthetics and such complex issues like sin, guilt and damnation.
In recent times, the novel has been read as one of the first instances of science-fiction as well though one would have to say that it also critiques the rationalist process of scientific creation, which does not take into its account, the zones of emotion or the affective space in general. Above all, it is a romance, quite tragic in character, and one may as well read into it a modern tale of human alienation, that is the destiny of the 'monster'.