Abstract: A simple viscosity method from Pal-Rhodes equation was described for the change of solution viscosity with time for one kind of coal soluble constituents PI-1, and the aggregation dimensions of the aggregates in solutions were calculated. The aggregation dimensions of PI-1 molecules in NMP and in CS2/NMP mixed solvent at 25 ℃ are 2.08 and 2.19, respectively, indicating the aggregation of PI-1 in solution is a reaction limited aggregation (RLA) mechanism. The aggregation dimension decreases with increasing temperature, suggesting the aggregation kinetic process may be converted to a diffusion limited aggregation (DLA) mechanism at higher temperature. The aggregation dimension of PI-1 aggregates is larger in NMP solution than that in CS2/NMP mixed solvent at high temperature (50 ℃), indicative of a lower aggregation kinetic process in NMP due to the lower diffusibility (mobility) of PI-1 molecules in NMP compared to that in CS2/NMP mixed solvent.