A suspended graphene oxide device is fabricated and investigated using a transmission electron microscope (TEM) scanning tunneling microscope (STM) setup. A detailed study of step-by-step reduction of an individual graphene oxide sheet under current flow and Joule heating in tandem with conductivity measurements, atomic structure imaging, chemical composition, and bonding alternations tracing is performed. As monitored by electron energy loss spectroscopy, the oxygen content is tuned from that peculiar to a pristine graphene oxide (i.e., 23.8 at %) to oxygen-free pure graphene. Six orders of magnitude conductance rise is observed during this process with the final conductivity reaching 1.5 �� 10(5) S/m. Quantification of plasma energy losses of the starting graphene oxide shows that ���40% of the oxygen atoms are in the form of epoxy, and ���60% oxygen atoms are in the form of hydroxyl. The total portion of sp(3) bonds in pristine graphene oxide is estimated to be ���45%. The epoxy groups show a larger influence on the conductivity of graphene oxide than hydroxyl ones. Through analyzing consecutive plasma-loss energy spectra under gradual graphene oxide to graphene transformation, it is found that the oxygen atoms in epoxy groups decompose prior to those in hydroxyl groups.