An antenna structure includes a feeding portion, a first grounding portion, a second grounding portion, a first loop antenna and a second loop antenna. The feeding portion has a first side and a second side parallel to the first side. The first grounding portion is positioned adjacent and apart from the first side of the feeding portion. The second grounding portion is positioned adjacent and apart from the second side of the grounding portion. The first loop antenna is defined to accept a second loop antenna therein, and is electronically coupled to the first and second grounding portions. The second loop antenna is positioned inside and apart from the first loop antenna, and further electronically coupled to the feeding portion.