This study investigated Korean middle school students’ acquisition of high-frequency derived words in English. A total of 36 pairs of base and derived word forms within the band of 1,000 most frequent words were extracted from the Corpus of Contemporary American English, and then a test set was developed to assess students’ morphological awareness and their ability to choose the correct word form in a given syntactic context. The main findings are as follows: First, Korean students tend to acquire the V-N and Adj-N types of derivation better than the N-Adj and Adj-Adv ones. Second, base forms are not necessarily acquired better and earlier than derived forms. Third, the productivity of a suffix does not predict its learning difficulty to a significant degree. Despite its high frequency and productivity, the suffix ?al turns out to be the most difficult item for Korean students. Fourth, acquisition seems to be more facilitated when the two morphologically related forms show a moderate degree of difference in frequency.