Reduced graphene oxide (rGO) sheets are synthesized and tunneling junction devices are fabricated with an aluminum oxide layer inserted in between electrodes and rGO sheets. Differential conductances, revealing density of states (DOS), of rGO sheets are measured in a wide voltage range. A difference in DOS of rGO sheets with different thickness is identified. For the single-layer rGO, the DOS shows a whole band with band edges in line with theoretical predictions, and gating DOS is used to estimate electrons Fermi velocity. Disorder effects on conductance and DOS of rGO sheets are explored and compared with each other.