Abstract: In this paper, it's argued that there are three sub-categories of the Mandarin Chinese modal particle“a”, namely, “a₁”, which is attached at the end of a sentence, “a₂”, which is a topic marker occurring topic — finally, and “a₃”, which is placed after some constituents between the topic and comment.The paper shows that “a” can be treated as three different particles, depending on the constituents that it is appended to. A hierachy of the three particles,i.e.“a₁”>“a₂”>“a₃”is also proposed,which is embodied in their functional and formal features,such as the constitutents they can be attached to,omission,subjectivity and mutual subjectivity,etc.