The structure of a p-xylene adlayer formed on Rh(111) in ultrahigh vacuum (UHV) at room temperature was investigated by using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). STM images revealed that p-xylene formed a highly ordered adlayer on Rh(111) in UHV. From the STM observation and the LEED measurement, the adlayer structure was determined to be c(2 root3 x 4)rect, which was reported previously also for the adlayer on Rh(111) in HF solution. Time-dependent STM allowed direct observation of the dynamics of the self-ordering process. Small ordered domains of adsorbed p-xylene molecules appeared and gradually grew in size on terraces. The domain size was found to increase in a few minutes under the exposure to p-xylene. The average domain size was ca. 20 nm.