This paper compares a MEMS Piezoresistive pressure sensor which utilizes a circular and square shaped polysilicon diaphragm with a nanowire to enhance the sensitivity of the pressure sensor. The nanowires for both the sensors form the bridge between the diaphragm assembly. The high Piezoresistive effect of polysilicon nanowires is used to enhance the sensitivity. The circular and square shaped polysilicon diaphragm with nanowire which are like piezoresistor were fabricated by means of RIE(reactive ion etching). This paper describes the performance analysis, structural design and fabrication of piezoresistive pressure sensor using simulation technique. The polysilicon nanowire has a thickness about 10nm. Finite element method is adopted to optimize the sensor output and to improve the sensitivity of both the circular and square shaped polysilicon diaphragm with nanowire to form Piezoresistive pressure sensor. The best position to place the Polysilicon nanowires to receive maximum stress was also considered during the design process. It was found that the fabricated square polysilicon diaphragm with nanowire has the highest sensitivity of about 156mV/VKPa were as the circular polysilicon pressure sensor gave a sensitivity of about 133 mV/VKPa.