Nasal Finals play an important role in distinguishing lexical meanings in Standard Chinese, but it is still unclear what the primary perceptual cues for nasal Finals are. The present study looks into this question, especially the primary perceptual cues for native Chinese listeners. We conducted two perceptual experiments with three-formant synthetic stimuli in which the second formant (F2) and the third formant (F3) were varied. Experiment I varied F2 and F3 simultaneously in the vowel part (including vowel nucleus and nasalized vowel). Experiment II varied F2 continuously, in parallel with varying F3 by step in the vowel part. Results show that subjects tended to perceive categorically in both of the two experiments. In addition, although F3 plays a role in the perception of velar nasal Finals, F2 is the primary perceptual cues for nasal Finals in Standard Chinese. This study not only steps further on the research of perceptual cues for nasal Finals, but sheds more light on phonetic teaching in the area of teaching Chinese as a Second Language (CSL).