Travelling outside USA on OPT

http://www.unh.edu/oiss/students/currentf1/opt.html

Travel Outside the U. S.
An F-1 student with an unexpired EAD issued for post completion-practical training and who is otherwise admissible may re-enter the US to resume employment after a temporary absence. The EAD must be shown in combination with a letter from the employer and an I-20 endorsed for travel and re-entry by the DSO.