Compared to the conventional ISFET device, the silicon nanowire ion-sensitive field-effect transistors sensors (Si-nw-ISFET) has attracted a lot of attention andis considered as one of the most promising candidates because of their biocompatibility, very high surface-to-volume ratiodue to the very small sizes of the nanowires, fast response, and good reliability of the signal. This paper deals with the modeling and performance evaluation of Si-nw-ISFET(Silicon-nanowire chemical fieldfield effect transistor) microsensor. The modeling approach used in this work takes into account the nanowire size effects to evaluate the response of Si-nw-ISSFET transistor and introduces two parameters: the total resistance of nanowire RT and the rectangular capacity Cox of the gate insulator all-around the Si-nw. This model investigate the main influential parameters of nanowire on the sensor response as: (i) different wire lengths Lnw, (ii) channel gate lengths of Si-nw-ISFET ( i.e. short and large gate length) and (iii) different numbers of parallel wires. The model developedis tested on large pH range, evidencinga good fit between simulation and experimental results.The model developed can predict the Si-nw-ISFET response behaviour and creates new opportunities for new innovative applications.