The evolution of precipitates size distribution of Ti-IF steel during annealing process has been investigated by simulating different continuous annealing (CA) and batch annealing (BA) treatments. The size and distributions of the precipitates (TiC in particular) are obtained by TEM observation and measured image analysis. They are then fitted by log-normally distribution and a model demonstrating the relationships of particle size distribution with annealing process parameters has been developed and validated by experiment results. The fine TiC precipitates (< 30 nm) vary after different annealing treatments. The effects of annealing temperature and holding time on the evolution of TiC particle size distribution are discussed.