This study is a case study of the effects of tonal L1 on the acquisition of tonal L2, comparing two groups of tonal L1 learners, namely, Chinese Zhuang (C+Z) and Chinese non-Zhuang (C-Z) in the same classroom. The two groups of learners read aloud 60 words from a Thai wordlist, then their tone production was analyzed in two dimensions. The phonological analysis focused on tone error patterns while the acoustic analysis focused on the fundamental frequency (F0) compared to the Thai native speakers. The Praat Program was employed for the acoustic part. The analysis of tone errors showed positively that the C+Z group had higher rates of accuracy than the C-Z group, arguably due to their familiarity with Tai tone patterns, with no confusion stemming from Zhuang tone patterns. Contrastingly, the phonetic characteristics findings showed negatively that the C+Z group shared less similar production with Thai native speakers group than the C-Z group's ones, arguably due to L1 interference. The current study confirmed both positive and negative effects of tonal L1 on the acquisition of L2 tones. Therefore, tonal problems should be investigated closely and identified clearly for the efficiency and effectiveness of tone acquisition training.