The invention relates to a wide-band planar antenna. The wide-band planar antenna includes a substrate, a first radiator, a second radiator, a third radiator, a ground, and a signal source. The first radiator, the second radiator, and the third radiator are designed in a manner that the antenna of the invention can be applied to WiMAX communication devices. Besides, the wide-band planar antenna of the invention is more efficient than a general wide-band antenna and saves a significant amount of electrical power, and therefore, the antenna is particularly suitable for portable communicational devices.