A composite structural panel for use in bridge structures, and method of manufacturing same, comprises a top panel and a bottom panel separated by and attached to at least one, but preferably a plurality, of structural composite preforms which may be fabricated by a continuous manufacturing process and may be saturated by resin using a continuous wetting process. The composite preforms may take any cross sectional shape but are preferably trapezoidal. The top and bottom panels may be fabricated from a plurality of layers of woven fabric layers and non-woven fabric layers which are saturated with a resin that is subsequently cured using cure processes known in the art. The composite structural panel of the invention is usable as a flat structural member for use as bridge decking, ramps, trestles, and any application requiring a structural panel.