We present nanometer-scale date storage on 5-phenyl-1-ureidonitrile thin film using scanning tunneling microscopy (STM). This monomer film can bt:come polymerized when a voltage pulse is applied between the STM tip and the substrate. The affected region changes from being electrically resistant to electrically conductive along the direction of the polymeric molecular chain. The recorded mark of the data occurs in the region, and is 0.8 nm. We have obtained recorded marks arranged regularly as 6 X 8 arrays. A possible chemical mechanism of this data storage would be the phase transition from the monomer state to the polymeric state. The small marks are very stable.