This paper describes two competing factors, a dielectric constant and a charge in a dielectric-modulated field-effect transistor (DMFET), for label-free DNA electrical detection. Essentially, the DMFET electrically detects biomolecules by monitoring a change of threshold voltage caused by a change of dielectric constant when targeted biornolecules are confined to a nanogap of the DMFET. In particular, when charged biomolecules such as DNA are introduced into the nanogap, the DMFET operation can be changed by both the dielectric constant and the strength of the charges in the gate dielectric layer. In this work, negatively-charged DNA and neutralized DNA by sodium ion treatment are carefully compared using an n-channel DMFET in order to verify the contribution to a change of threshold voltage by the DNA charges. In the case of neutralized DNA, the threshold voltage is shifted to the negative side as previously reported. However, in the case of negatively- charged DNA, the threshold voltage is shifted to the positive side due to the negative charges of this DNA. Hence, a p-channel DMFET is clearly preferable in detections of negatively-charged DNA.