Nb (CN) particle in Nb-HSKA steels plays important role to retard recrystallisation process and to strengthen the mechanical properties by precipitation hardening. The behavior od NB (CN) particle growth after deformation needs to evaluate in order to optimize the production process. HSLA steel containing 0.03% Nb is used and the thermomechanical treatment including reheating air cooling, finishing and relaxation is applied to the test steels. The results obtained in this way shows that the kinetics coarsening of NB (CN) particle after deformation at 900C is considerably accelerated compared with when the coarsening is controlled by a bulk diffusion process.