Guanine nucleobases in DNA (GDNA) has a strong binding affinity towards lead (II) ions. Functionalized with graphene field-effect transistor (GFET), it makes an ideal sensing element for GFET-based sensor. The sensing is typically observed in the transfer characteristics of the GFET. Upon successful binding of GDNA with Pb2+, the Charge Neutrality Point (CNP) of the GFET will be right-shifted accordingly. This paper aims to evaluate the total charge density and single-stranded GDNA needed for the CNP shift to take place. This is achieved by simulating the GFET transfer characteristics using drift-diffusion model in MATLAB. The simulation takes into account graphene quantum capacitance, channel capacitance and related charges including the captured Pb2+ ions. It reproduces correctly the published transfer characteristics of GFET. Moreover, it also can estimate the CNP shift, number of single strand DNA needed, and number of ions captured. Knowing such features are essentials in enhancing the sensitivity and limit of detections of the GDNA-GFET sensor. © 2019 IEEE.