Coal organic microspheres (COM), a novel material originated from the organic components of coal, were obtained from brown coal directly using a one-step treatment in a flow of water. Here, we studied in detail the effect of hydrothermal conditions on morphology, yield, elemental composition, functional groups, and molecular composition of COM, at temperatures of 250, 300, 350, and 380 degrees C, pressures of 4, 10, 20, and 24 MPa, and holding times of 0, 30, 60, and 90 min. The holding time influences COM production: longer holding time leads to higher yields and larger particle size. The hydrothermal temperature and pressure, which control the properties of water, have a significant influence on the yield, shape, and particle size of COM. The properties of water affect the COM yields as follows: 1) when the phase of water is liquid, the lower density, viscosity, and static dielectric constant gives higher yields; 2) the yield obtained in supercritical water is higher than that in subcritical water; 3) phase transformation of water from liquid to gas decreases the yield. Additionally, the particle size of COM can be controlled by the temperature, and the pressure affects the shape of COM greatly; even nanosized particles are produced at 350 degrees C and 10 MPa. From the molecular composition results, low-molecular weight compounds with small double bond equivalents decompose under supercritical water condition. Large amounts of organic compounds in brown coal are thermally decomposed and extracted under hydrothermal conditions, and then spherical COM particles form and grow from small to big during the cooling process. These results are helpful for clarifying the formation mechanism of COM, and enable control of COM yield and properties.