Abstract: The former studies concerning middles did not reach agreement on the syntactic and semantic feature of this structure. Starting from the structural differences between passives and middles, this study attempts to prove that there exists neither explicit nor implicit external argument in middles. Semantically, the absence of the external argument is caused by the conceptual eventiveness feature of the main verbs. Middles ought to contain adjuncts indicating property and mode. The prepositional phrase possibly existing in middles is adjunct. In addition, the present study also defines and classifies English and Chinese middles based on structure.