Hi Ramin Although my answer may not seem relevant to your question, I share my experience to you. I have taken TOEFL NOT IELTS. However in my personal experience, the best way to prepare for such exams is to use simulator software. I mean the software by which you can experience what will happen in the test session. You can find these softwares by google. If you're supposed to memorize lots of words, I think it won't be too useful. Because IELTS doesn't measure how many words you know. Instead, It evaluates how well you speak, write, understand audible passages and written texts. Hope this helps Good luck

Hi ramin,i recommend you to study (4000 essential english words)