Ultrahigh-density data storage on a 3-phenyl-1-ureidonitrile (PUN) thin film was performed using a scanning tunneling microscope. The recorded marks of 0.8 nm in diameter were obtained when voltage pulses of 4 V for 10ms were applied between the STM tip and highly ordered pyrolytic graphite substrate. The current-voltage relations at the local regions of the films indicate that the recorded region is conductive and the unrecorded region is in a high impedance state. A possible mechanism of this data storage was suggested and discussed.