I am preparing Japanese resume and i want to mention my skills in 特技。Is “勤勉” a correct word to say that i am hard working???
Yes 勤勉 is a common na-adjective that means "hardworking", and you can consider using it in the 自己アピール/自己紹介 section (or something with a similar name).
However, it usually does not count as your 特技 because 特技 refers to some uncommon/special ability. Writing just 勤勉であること or 努力できること in the 特技 section might look as if you were a boring hobbyless person. In your case, probably you can list at least 英語 as your 特技. You can list your other abilities/hobbies such as 野球, 料理, プログラミング, 数学 or whatever that may interest the interviewer. (Well, if you can add some interesting episode that can prove your outstanding hardworking-ness, 勤勉さ or 集中力 might count as your 特技...)