Perceptually distinguishing between Mandarin alveolar nasal coda [n] and velar [η] are difficult for Japanese natives in learning Chinese as a second language (CSL). Discovering relations between acoustic cues and perceptual responses is important for studying CSL acquisition and computer-aided pronunciation teaching. In order to investigate the influences of nasal coda's lengths on nasal perception by Chinese and Japanese, two studies were conducted. One is a statistical comparison of Mandarin nasal codas' durations. The other one is an identification experiment in which subjects perceive stimuli with gradually-shortened nasal codas. Results reveal that the difference between durations of [n] and [η] is non-significant. Furthermore, nasal codas' length hardly affects Chinese subjects to identify nasal type, but it is a relatively great impact for Japanese. Slightly more correct responses are obtained when Japanese identify stimuli with longer codas, and those with shorter endings are more likely to be identified as non-nasals.