In this paper, a fractal ultrawideband multiple-input–multiple-output antenna is proposed and its characteristics are studied in free space as well as in the proximity of on-body wireless body area network scenario. The performance of the wideband antennas are affected strongly in the proximity of the human body. Koch fractal geometry is used at the outer edges of basic octagonal geometry in the design, which helps to achieve desired miniaturization, wideband and stable radiation pattern. The presented antenna has a compact size of 25 mm × 40 mm. In addition, the effect of body tissues on antenna characteristics is analyzed by a four layered (skin, fat, muscle and bone) human tissue model. The comparative analysis of S21 is performed at a various body parts position such as abdomen, near forehead and chest with different antenna orientations. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.