The effect on free radical concentration of chip surface treated with saturated steam at elevated pressures was investigated in this study. The ESR spectroscopy of wood chips of spruce ands birch after steaming under various pressures and time was analysed. The results, showed, a)after 1.275N/mm~2 pressure and 5 min. steam treatment, the free radical concentration of chip surfaces was higher than that of untreated, b) a reduction of 2% in resin content of the particleboards made from steamed chips resulted in reduction, in thickness swelling as compared to the controlled particleboards because of the change of the free radical concentration of the chip surface.