The purpose of this study was to describe the bilingual child's ability and challenges in speaking and writing in two languages: Bahasa Indonesia and English. This research was designed by using a case study. The subject of this research was an 11-year-old, who communicated in both Bahasa Indonesia and English. Data were collected through observation, interview, video recording of the child’s speaking English and Bahasa Indonesia, and document analysis. The result showed that the subject was an active bilingual child because he actively spoke Bahasa Indonesia as his mother tongue and English as his second language. In English speaking, he had constraints in pronouncing /wɑː.zənt/, /bɑːt/. In speaking Bahasa Indonesia, he missed spealing some words, such as Hari, Dermaga, Bermain. In, writing English, he had difficult in writing words. He dominantly had challenge in writing the Indonesian Language, especially in developing content, vocabulary used, and in grammar used.