Previous studies showed that bilinguals tend to score lower on some language tests than monolinguals in each of their languages and outperform monolinguals on some cognitive tasks. We investigated whether short-term and working memory capacities that underlie language abilities differ between sequential bilinguals and monolinguals. We tested 4-6-year-old English monolinguals' and English-Mandarin sequential bilingual's receptive vocabulary, as well as predictors: (1) age, (2) non-verbal intelligence, (3) verbal short-term memory, (4) verbal working memory, and (5) visuo-spatial working memory. Mandarin monolinguals of the same age were also included and tested on vocabulary, verbal short-term memory, and visuo-spatial working memory. The bilinguals' vocabulary scores were lower than the monolinguals' in both languages, although much lower than the English monolinguals. Bilinguals' vocabulary scores showed stronger correlations with working and short-term memory than the monolinguals'. These results suggest that learning another language might impose different memory processing demands than learning a native language.