In the context of learning English in secondary schools and high schools in Vietnam in general and in Mekong Delta in particular, writing is usually considered the most challenging skill for the majority of students. These students frequently encounter a lot of obstacles in their writing practices, such as high-standard conventions in academic writing, complicated  grammatical structures, lack of ideas, an insufficient amount of time spent on writing in class, as well as poor and noncomplex vocabulary. Such challenges do not only result in low performances and low scores but also students’ fear and loss of interest in writing lessons. Several previous studies indicated that the teaching of morphology had positive impacts on expanding students’ vocabulary, and indirectly improved the quality of their writing. This current study was conducted in order to examine the impacts of teaching morphology to EFL intermediate students on the degree of lexical complexity and the quality of their academic essays. This study employed the experimental research approach, which utilized three different research instruments: writing tests (before and after the study), questionnaires (before and after the study), and focus group interviews (after the study). This 14-week study was carried out at a private English center in Mekong Delta, with the participation of two groups of EFL students who were studying at the same level and progress. The results revealed that the instruction of morphology to EFL intermediate students had positive impacts on the degree of lexical complexity and the quality of their academic essays. Moreover, the results of the questionnaires and interviews also pointed out that these students had positive attitudes towards the instruction of morphology in their academic essay writing lessons. Finally, several recommendations and pedagogical implications were concluded with the attempt of contributing to the innovation of teaching vocabulary and teaching writing in the context of secondary schools and high schools in Mekong Delta particularly and Vietnam generally.