This paper discusses the grammaticalization of motion verbs in Mandarin. A class of motion verbs in Mandarin that regularly appears at either V1 or V2 position in the V1+V2 construction is only grammaticalized at the V2 position, where the verb becomes a directional complement. It provides a cognitive semantic account and proposes a new hypothesis that is called the syntactic position and event type sensitivity hypothesis in grammaticalization. The paper analyzes corpus data across five historical stages for 11 simplex directional complements. The analysis draws on Talmy’s macro-event theory and Lehmann’s grammaticalization parameters. It is concluded that motion verbs at the V1 position are most likely to have agentive subjects, which foregrounds the idea of motion in V1, while V2 focuses on the Agent’s purpose. Motion verbs at V2 are relatively more likely to have non-agentive subjects, which foregrounds the Path element in V2 and complements the action of V1, rather than the purpose of the Agent. What triggers the grammaticalization of the V2 is the foregrounding of the Path element in V2, which complements the action of V1, and its non-agentive subject.