This work was undertaken to systemically study the effect of various process parameters (solvent type, reaction temperature, hydrogen pressure, coal rank, and coal particle size) on the rheological properties of coal-derived liquids (CDLs). Among three investigated solvents (tetralin, a medium oil, and a heavy oil), only tetralin enhanced the coal conversion because of its superior hydrogen-donor capability. The viscosity of the CDLs exhibited a strong positive correlation with that of the solvent used. The increase of either reaction temperature or hydrogen pressure decreased the viscosity of the CDLs. A comparison of coal rank illustrated that a bituminous coal resulted in higher viscosity of the products than a subbituminous coal. The experiments performed with three different coal particle sizes (-60, -100, and -200 mesh) showed that coal with a smaller particle size distribution generated lower viscosity liquid products. It can be deduced that the coal liquefaction process parameters have a strong effect on the chemical compositions of CDLs and, therefore, their physical properties, such as viscosity. To accurately predict CDL viscosity, preliminary work was conducted to correlate the viscosities of CDLs generated at different conditions with other CDL physical properties [e.g., boiling point and American Petroleum Institute (API) gravity] using three commonly used petroleum viscosity correlations. The results showed that the predicted viscosities were much lower than the measured ones because of the presence of stronger hydrogen bonding in CDLs than in petroleum fractions. A simple correction could improve the viscosity prediction for the CDLs generated from tetralin by reducing an absolute average deviation from 6.5 to 28%.