Abstract: In Hallidayan sense， the rankshift of grammatical metaphor is unidirectionally downgrading. In recent years， however， Chinese systemicists have proposed the bidirectionality of rankshift. This research finds that rankshifts take place along two dimensions， one is the transfer from semantics to lexicogrammar in the content layer of language and the other is that among grammatical categories at the lexicogrammaticical level; both are unidirectional. Ideational metaphor is manifested as downward rankshift， metaphors of modality as unidirectional transcategorization from modal verbs to modal adverbs or prepositional phrases and finally to modal projecting clauses， metaphors of mood as unidirectional transcategorization from unmarked to marked mood choices， and textual metaphor as unidirectional transcategorization from unmarked to marked structures. Therefore， the bidirectionality of rankshift does not accord with the mechanism of grammatical metaphor.

Key words: grammatical metaphor, rankshift, transcategorization, transfer dimension, unidirectionality