The high transconductance and low noise of graphene-based field-effect transistors based on large-area  monolayer graphene produced by chemical vapor deposition are used for label-free electrical detection of DNA hybridization.The gate materials,buffer concentration and surface condition of graphene have been optimized to achieve the DNA detection sensitivity as low as 1pM(10^-12M), which is more sensitive than the existing report based on few-layer graphene.The graphene films obtained using conventional PMMA-assisted transfer technique exhibits PMMA residues,which degrade the sensing performance of graphene.We have demonstrated that the sensing performance of the graphene samples prepared by gold-transfer is largely enhanced (by125%).