This study investigated students’ perception of learning effects of AI-based English writing learning tools, Google Translate (GT) or Naver Papago (NP) and Grammarly, and their strengths and weaknesses. A perception survey about using those AI tools in English Writing was conducted with 153 students majoring in Engineering. Results showed that both GT or NP (GT/NP) and Grammarly were helpful for students to learn English writing. However, their perception of Grammarly was statistically better than that of GT/NP. Many students’ responses on strengths and weaknesses of each tool showed that: (1) GT/NP was much helpful in learning vocabulary, grammar, and sentence structure, but inaccurate translation was an issue; (2) Grammarly was much helpful in checking and correcting errors without a major drawback; and (3) excessive use of AI-based English writing learning tools could interfere with their learning English or English writing. Implications from this study are as follows. When learners use AI-based learning tools for English writing, classes need to be designed so that they can use them effectively rather than rely on them. How students with varying levels of English writing use AI-based learning tools also needs to be examined.