This inquiry tested the Perceptual Assimilation Model (PAM) on its applicability in child L2 lexical tone acquisition. The potential effect of L1 (Cantonese) lexical tones on L2 (Mandarin) lexical tone learning was examined. Precision rate and error patterns were explored with an AX discrimination task and a forced-choice identification task. Forty-nine native Cantonese-speaking students aged 8 years partook in the inquiry. Results disclosed that these children displayed almost perfect performance in the discrimination of Mandarin tones. However, significant tone differences were discovered in the identification task. Tone 4 (T4) was distinguished with the lowest precision, and T1 with the highest. Error analysis disclosed that Mandarin T2-T3 was the most confusing pair, followed by the T1-T4 pair. The inherent phonetic similarity between lexical tones in a language and the tone resemblances across languages may also have contributed to perception troubles, which could help to refine and supplement the PAM in the tonal/suprasegmental domain.