As there is no vowel length contrast in Chinese, Chinese learners of Thai (CLTs) often have difficulty distinguishing between short and long vowels, especially when the same vowel grapheme in Thai orthography can be pronounced as both a short and a long vowel. This study examined the importance of metalinguistic knowledge (MLK) used by CLTs before and after praxis intervention with the aim of providing insight into the relationship between MLK and L2 acquisition. Twenty CLTs and ten Thai native speakers were tested for their ability in identifying Thai vowel length. The CLTs were divided into two subject groups following the pre-test. The first group (CH1) was given only exercises, while those in the second group (CH2) were also taught rules. The two groups were tested for accuracy and reasoning behind a given identification. Two-tailed t-test results show statistically significant differences in scores of CH1 and native speakers and a positive relationship between accuracy in identification and L2 learners' MLK, including the ability to identify, describe, and explain L2 vowel length. Importantly, CH2 performed better in the post-test than those in CH1, suggesting that MLK helps enhance L2 acquisition. In addition, the results of a mixed-effect regression model show that four factors (group, stimuli type, tone, and structure) influenced the identification scores of CLTs.