This study reports the results of three experiments on Korean simple vowel perception in order to examine the effect of similarity in second language sound perception. A total of 72 native Cantonese college students who learned Korean as a foreign language in Hong Kong participated in three experiments designed according to the frame of the Perceptual Assimilation Model (PAM). The results of Experiment 1 indicate how Korean and Cantonese simple vowels are mapped in initial learning stage. The findings of Experiment 2 verify the validity of the PAM in Korean-Cantonese language pairs, suggesting that two similar sounds are difficult to distinguish. Experiment 3 was a longitudinal study to support the PAM with empirical data. Suggestions for teaching methods are provided to enhance Cantonese speakers` perception of Korean simple vowels.