To obtain a fiberboard with large strength and excellent dimensional stability by opening stems of hemps such as kenaf at a specified opening temp.

Stems of kenaf which is one of hemps are steamed at 100-160°C and are opened by means of a pressure refiner adjusted to clearance of 1.3 mm and temp. of 100°C to obtain fibers with water content of 80%. While the fibers obtd. are stirred by means of a stirrer, 12% (wt.%) urea resin adhesive to oven-dry fibers and 1% paraffin wax as a water resistance providing agent are added. The mixture of the fibers, the adhesive and paraffin is dried to water content of 10% and it is formed into a fiber mat by means of a vacuum former. The mat is preliminarily press-clamped and is hot press-molded into a sheet by means of a hot press to prepare a fiberboard. The fiberboard prepd. by using the fibers prepd. by opening the stems of kenaf at 100-160°C exhibits extremely excellent performances in all of bending strength, peeling-off strength, expansion ratio by water absorption and whiteness in comparison with those for ordinary fiberboards.