The irreversible thermal aggregation rate and process of bovine serum albumin (BSA) were investigated by means of light scattering technique as a function of temperature. The increasing rate of particle radius was affected by the aggregation temperature, concentration and the presence of fatty acid. The particle radius was larger and the aggregation rate was faster for fatty acid free BSA at higher temperature and concentration. Two thermal aggregation processes were observed at relatively low temperature and concentration, both for fatty acid containing (C-BSA) and fatty acid free BSA (F-BAS). The first process proceeds by an inter-monomer aggregation mechanism, and the second process by inter-aggregates aggregation. The first process is represented by a power law as Rhapp∝tz, which is diffusion limited cluster aggregation (DLCA).