A study on the role of intertidal buffer zone on the purification of polluted nearshore water was performed. Some gravel samples were taken from a intertidal zone, and their microbial activity was estimated using an aerobic respirometer system, and compared with that of suspended marine microorganisms existed in a near shore water. The maximum oxygen uptake rate of the suspended marine microorganisms, indicating the potential of purification of polluted near shore water was 0.15mg O2/L/hr. For the gravels from the intertidal zone, the maximum uptake rates of oxygen were affected by the vertical positions, but their gross value were around 0.77mg O2/L/hr, which was around 5 times higher than the purification potential of polluted near shore water by the microorganisms existed in the near shore water. This indicates that daily purification ability of the gravel in the intertidal zone is amount to around 18.5 mg BOD per cm2 of gravel surface. However, microbial activity of the gravel and their purification ability of pollutants were severely affected by inflow of fresh water. As amount of fresh water flow into the sea water was increased to 10% of the total polluted water, the microbial activity, as well as the purification ability of pollutants was severely decreased, and at over 20%, they were approached to the minimum value. The microbial activity of the gravel was also affected by the concentration of pollutants. When the concentration of pollutants exposed to the gravel was increased to 10 times of the normal concentration, the oxygen uptake rate, indicating microbial activity of the gravel for degrading of organic pollutants, was not so much different, but the total oxygen consumption was 1.6 times at the normal value. This indicates that the gravel intertidal zone plays an important role in controlling the non-point source pollutants from land, as well as self-purification of polluted near shore water by trapping and degrading the particulate organics.