We conducted a three-month field experiment focusing on the physical and chemical characteristics of fog in a tropical rainforest in Xishuangbanna, Southwest China, in the winter of 2019. In general, the fog would form at midnight and persist because of the increased long-wave radiative cooling combined with the high relative humidity, gentle breeze, and a relatively low aerosol number concentration in the forest; the fog would dissipate before noon due to the increasing turbulence near the surface. This diurnal cycle is typical for radiation fog. The microphysical fog properties included a relatively low number concentration of the fog droplet, large droplet size, high liquid water content, narrow droplet number-size distribution, and high supersaturation. The chemical properties showed that the fog water was slightly alkaline with low electrical conductivity, whereas the highest proportions of anions and cations therein were Cl- and Ca2+, respectively; the chemical components were enriched in small fog droplets. In addition, we indirectly calculated the fog supersaturation according to the kappa-Kohler theory. We found that condensation broadens the droplet number-size distribution at relatively low supersaturation, which is positively correlated with the fog-droplet number concentration and negatively correlated with the droplet mean-volume diameter; this affects the key microphysical processes of fog.