Again, see what the fair sponsor says about attire on their web site or other promotional materials. If they don't tell, contact the fair sponsor and ask. Stand up straight, don't hang your mouth open, don't fidget, do speak up and speak clearly, don't chew gum or smell like smoke. Have a good handshake and make good eye contact. Don't be misled into thinking of the fair as a social event .
Employers often send recently hired new graduates to career fairs.
Don't fall into the mistake of interacting on a social level and forgetting that you are being judged on your potential to function in the work environment. Carry a simple padfolio to keep your resumes organized and ready . Many fairs have you check your bags at the door because the event is crowded. Bottom line is that you want to look like an organized person because that's an asset in an employee. Have an open mind.