The continuous miniaturization of sensors, as well as the progression in wearable electronics, embedded software, digital signal processing and biomedical technologies, have led to new user-centric networks, where devices can be carried in the user’s pockets, attached to the user’s body. Ultra wide band (UWB) technology is an innovative wireless technology which can transmit digital data over a wide frequency spectrum with very low power and a very high data rate. Actually, wireless body sensor networks comprise with sensor nodes that is very adjacent to the body on or in daily wear. It is mainly advantageous applications to the biomedical, such as human body probing, health monitoring, real-time diagnosis etc. Ultra Wideband (UWB) has enormous prospective for applications in the wireless body area networks (WBANs). Designing an efficient antenna with UWB technology is a great initiative as an application of WBAN. To design an efficient antenna with band-notch technology is a good key. This paper offers the antenna design & performance study of Ultra Wideband band-notch antenna for the wireless body sensor networks. Both the free space & the on-body behaviour of the antenna with FR4 substrate is investigated and analysed. After the antenna modelling in free space, it is modelled on human test phantom to study the on-body performance. The free space and on-body performance of this antenna were compared and it shows very good performance. For the better comparisons, important antenna factors such as gain, return loss response, efficiency & radiation pattern are closely analysed. The antenna shows good on-body behaviour & proper band-notch characteristic and as a result it will be appropriate candidate for wireless body sensor networks for medical applications. Moreover body-centric applications of the antenna such as GPS, healthcare, military applications and fire fighter personal communications are at peak demand as well.