A feasibility study of wastewater treatment by downflow biofilter using sea shells as filter media was investigated. The experiments were performed on 5-10 mm diameter sea shells with 2.52 g/cm3 density and a synthetic wastewater with 300 mg/l COD and COD:N:P controlled ratio of 300:40:10. The experiments were carried out in five sets in five sets. The first and the second sets were to find out the appropriate media depth to be subsequently tested in the third to fifth sets. The hydraulic loading rate of the first 2 sets tested at 1.5 and 2.0 m depth was 0.62 m/hr. The results from these tests showed that a more efficient depth of filter media should be 1.5 m. The COD removal efficiencies were 97 and 95 percent, respectively. The TKN removal efficiencies were 96 and 90 percent, respectively, while the TN removal efficiencies were 30 and 28 percent, respectively. The subsequent last 3 sets were tested at 1.5 m media depth and at the hydraulic loading rates of 1.08, 1.54 and 2 m/hr. The COD removal efficiencies were 95, 88 and 80 percent, respectively. The TKN removal efficiencies were 54, 24 and 19 percent, respectively, whereas the TN removal efficiencies were 23, 23 and 18 percent, respectively. This indicates that the increase of organic loading rates from 3 to 10 kgCOD/m3-d did not much affect the system efficiency and the process could still produce an efficient COD removal, but the TN reduction rate was clearly reduced.