This paper reports research findings of English vocabulary acquisition of bilingual learners at the levels of Primary 6 and Secondary 3 at Satit Bilingual School of Rangsit University. The purpose was to find out the extent to which learners at these levels have acquired English vocabulary to communicate their ideas about themselves and their school life. The subjects were 34 Primary 6 students and 18 Secondary 3 students. All subjects were individually interviewed by two bilingual researchers of Thai and English: one Thai and one American. A set of ten questions was used in a 15-minute interview in English to secure lexical data or words from each subject. Vocabulary acquisition was assessed via communication skills at five levels: 1) Full control, 2) Functional control, 3) Moderate control, 4) Sufficient control, and 5) Marginal control. All interviews were recorded with consent of the subjects. During each interview, two more bilingual researchers of Thai and English were present to collect spontaneous speech data on words used by each subject. The obtained data were analyzed in frequency and percentage. The major research findings indicate that those subjects at the level of Primary 6 performed at five levels with a majority at level 2. The subjects in secondary 3 performed at three levels (1-3) with a majority at levels 1 and 2; there was none at level 4 or 5. The subjects at the level of Primary 6 and Secondary 3 show similar lexical features at specific levels with some variation in each, depending on the meanings individual subjects would like to convey in responding to the interviewers. It was noted that the subjects with three years' exposure to language immersion performed dominantly at level 2 and those with less exposure in years performed at levels 3 and 4.