This paper describes the designing of a coaxially probe fed square microstrip antenna (SMSA) used for RF energy harvesting system. A SMSA is designed with resonant frequency of 942MHz (GSM-900). The coaxial feeding mechanism is used due to its simplicity. Perfect tuning and axial ratio (AR) bandwidth (BW) in the design are achieved by optimizing the position of coaxial feed and dimensions corner truncation and slit respectively. The simulated impedance BW (VSWR=2) of 35 MHz (925 MHz-960 MHz, 3.71%) has been noticed. The ARBW achieved for the proposed antenna is 10.4 MHz (928.5 MHz-943 MHz, 1.1%) which is sufficient and within VSWR=2 band. Also, the simulated results of voltage multiplier and rectifier circuit along with matching circuits are described in detail in this paper. Good agreement between simulated and measured results has been noticed. The presented result shows that the proposed antenna system is practically applicable for RF harvesting and hence, prototype development is in process.