EL1 learners' receptive morphological words (root, inflected, and derived words) develop on different scales, but whether they develop similarly in EFL learners' language production is still unknown and deserves an examination. The answer may provide a clue to theoretical controversy about whether language learners' morphologically defined words are identical in identity and learnability and whether their productive vocabulary grows constantly. This study uses lexical frequency profile analysis to explore the developmental trends of advanced EFL learners' (n = 299) morphologically defined words in writing across four consecutive semesters. The results indicate that root words are much more significant in both amount and percentage in all four semesters than inflected and derived words, and infected words are much lower than root words but greater than derived words. Over time, the three morphologically defined words go along different development trajectories.