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Nitrate is a widespread groundwater contaminant, which can cause pollution of receiving waters. Some of the highest losses of nitrate to surface waters come from drained agricultural land. Bioreactor filters are a useful approach for removing nitrate from drainage waters, but these systems require an external carbon supply to sustain denitrification. The ability of barley straw to serve as a carbon substrate for biofilters was evaluated in a laboratory model. In this study the effect of two heads (100 and 200 cm) and two thicknesses of bioreactor (300 and 600 mm) were also evaluated. The experiment was conducted in the polyethylene columns with 90 mm internal diameter. The influent nitrate concentration was 40 mg L-1. Addition of barley straw as a carbon source decreases significantly effluent nitrate concentrations. The rate of denitrification was affected by the water velocity and decreased at velocity about 0.04 m h-1. In the columns with 300 mm height, the average nitrate reduction at 100 and 200 cm heads were 63.49% and 60.22%, respectively. The average nitrate reduction at the 600 mm height columns with 100 and 200 cm heads were 69.97% and 67.1%, respectively.