<p>Social comparisons (comparing their achievements with peers) and dimensional comparisons (comparing their achievements across academic domains) play important roles in shaping students’ learning motivation. However, how these processes shape students’ motivational beliefs (i.e., self-efficacy, interest, and utility) in learning two languages remains underexplored. Based on data from 802 bilingual elementary students enrolled in Chinese-English bilingual programs in non-traditional international schools in the Chinese mainland, this study examined the roles of social and dimensional comparisons in shaping their L1 Chinese and L2 English motivational beliefs. Results show that social comparisons existed, with higher Chinese/English achievements enhancing motivational beliefs in the corresponding domain. Dimensional comparisons affected self-efficacy and interest but not utility, with higher English achievement negatively impacting Chinese self-efficacy and interest. Furthermore, self-efficacy fully mediated the effects of achievement on interest and utility within and across domains. These findings can deepen our understanding of the formation of students’ bilingual learning motivation and provide insights for bilingual education practices.</p>