The present study investigated how well elementary 6th graders in Korea have learned their curriculum-based English words. In addition, the study tried to ascertain how far the participants had learned depending on the amount of instruction by recruiting the participants from different school systems. With Wesche and Paribakht`s (1996) vocabulary knowledge scale, a total of 36 words from 6th grade English textbooks were tested. The results revealed that first, all participants had slightly higher productive vocabulary knowledge than receptive vocabulary knowledge. Second, while the private school groups had more productive vocabulary knowledge than receptive vocabulary knowledge, the public school groups had more receptive vocabulary knowledge than productive vocabulary knowledge. Third, some words were similarly less-acquired, regardless of the amount of instruction. Fourth, the two groups showed similar errors at both receptive and productive levels. In conclusion, both groups` vocabulary achievement was statistically different, presented by vocabulary development levels; however, they revealed qualitatively similar erroneous features when they use the target words. These findings may have implications for English vocabulary learning and teaching, along with word-selection of English textbooks.