One of the most important problems of drainage systems is leaching of nutrients such as nitrate. Since this element is not enough in the soil, farmers have to use nitrogen fertilizers to provide plant , s demand. Nitrate leaches from the soil very quickly and it will bind to groundwater or surfacewater if drainage system exists. To reduce nitrate in the drainage outlet the denitrificator bacteria can be used. One of the conditions that can be effective on denitrificator bacteria activity is unsteady flow. In the present study barley straw is chosen as biofilter. The drainage model is made of galvanized sheet with 135 cm height, 30 cm width and 90 cm length. There were some holes with diameter of 5 mm in the inner wall. Water penetrated to the model from these holes. Barley straw was placed around the corrugated drainage pipe (12 cm diameter and 30 cm length) after combining with the soil. To evaluate the resistance of soil layers around the drainage pipe 10 piezometers were horizontally installed into side of them. Water with nitrate concentration of 80 mg/l during 18 periods (2 months) was entered into the model. Water samples were collected from the effluent and influent of the model during the course of experiments and then were subjected to nitrate, ammonium and pH measurements. Water sampling was performed simultaueously in four statuses (full, semi-full, 20 cm and 10 cm above the drain). The result represents the removal nitrate percentage increased and in four statuses (full, semi-full, 20 cm and 10 cm above the drain) are 47.5%, 50%, 52.5% and 80% respectively. The statistical analysis result for nitrate represented that there was significant difference between measurement periods and various water surface drop (p 0.01). For ammonium the difference was significant between various water surface drop (p 0.01) and no significant between measurement periods. Although ammonium concentration was very low (less than 0.03 mg/l) in the effluent. The pH values fluctuated between 7.00 and 8.10. Measurement of piezometer , s head during the study showed that the use of this biofilter around the drainage pipe has no adverse effect on hydraulic performance of the drainage pipe. According to this study, the barley straw is a suitable biofilter in case of reducing outlet nitrate from drainage pipes in unsteady flow. Key words Nitrate, biofilter, denitrification, drainage water, barley straw