Thermo-simulation test, TEM and nanobeam EDS technique were carried out to investigate the precipitation behavior of Nb, Ti, Mo etc in a HSLA steel. The strain induced precipitation occurred during isothermal relaxation stage after deformed in non-recrystallization temperature region. The results were compared with the refinement effect of microstructures caused by relaxation-precipitation controlling transformation (RPC) processing. The experiment results indicate that after 30% predeformation at 850degreesC and 900degreesC, there are two kinds of particles, (Ti,Nb)(C,N) and a few Nb(C,N), to precipitate during holding. The content of Nb in particles rises with the relaxation time increasing. During the final holding stage, some Nb and Ti atoms in the lattice sites of the precipitates will be replaced by Mo atom, the Mo content in the precipitates increases with the relaxation time. The shape of the precipitates is irregular, the distribution density and average size of precipitates are related to the deformation. The refinement degree of microstructure after RPC process is directly related to the amount and size of precipitates. The finest bainitic packet can be obtained when the average particle size is about 10nm.