We directly compared the effectiveness of a spelling intervention focused on morphological structure with one that emphasized the meanings of complex words, to differentiate their relative contributions to spelling acquisition in grade 3 and grade 5. We found that the morphology intervention provided a greater improvement than the vocabulary intervention, especially for children in grade 5. To compare the long-term effects of the two interventions, we tested the children’s spelling ability six-months after the conclusion of the intervention program. Results show that both grades maintain an increase in spelling accuracy compared to their pre-intervention performance. Additionally, the children in grade 5 who received morphological instruction retained more spelling knowledge than those who received the vocabulary instruction. These results suggest that teaching children about the structure of complex words supports their spelling ability in the long-term, providing evidence for the important role of morphological knowledge in literacy development.