In this paper a simple printed slot antenna with an enhanced bandwidth based on self-complementary structures is presented for UWB applications. The proposed antenna consists of a square radiating patch with a microstrip feed-line defected by an inverted U-shaped slot on one side, a rectangular slot and a defected ground plane by an inverted U-shaped strip protruded from rectangular slot on the other side. The antenna provides a wide usable fractional bandwidth of more than 100% (3.2-10.5 GHz). This large operating bandwidth is obtained by choosing suitable dimensions for the proposed quasi-self-complementary matching circuit. The designed antenna has a small volume of 20x20x0.8 mm(3). It exhibits almost omnidirectional radiation pattern with low cross polarization level.