L2 essay writing depends heavily on resources such as inputs from prewriting tasks, teachers, or dictionaries. The availability of Internet-based paraphrasing tools recently has offered learners a rich resource in terms of structures and vocabulary. The study explores how these tools help learners develop lexical and structural complexity in their written language. Totally 10 senior English majors from a college were allocated into two groups to write a 200-to-250-word essay every week during a course of 6 weeks. One group was instructed to use Wordtune – an AI-powered paraphrasing tool to revise their essays in the following week. The result from quantitative data shows no significant difference in lexical diversity while structural complexity of the group with Wordtune has increased slightly over time. Qualitative data from a week-by-week timescale also show how learners utilized and internalized the structures form the tool. Overall, the participants all felt positive about the tool.