Wheat straw waste materials were processed in a fully equipped pilot-scale MDF-process. Hammermilled wheat straw was used as a raw material in combination with a commercial melamine modified Urea Formaldehyde (UF) resin. Approximately 15 wt % of the wheat straw particles smaller than 0.7 mm was removed in a sifting operation. Medium density fiberboard was produced in the range of 790 to 860 kg/m3 average densities and at a resin content of 12.5, 13.1 and 14 wt %. The panels produced were approved according to the European Standard for MDF (EN 622-5:1997). Four different wheat straw fractions were investigated and sifted at 1.0 mm, 0.6 mm and 0.2 mm screen hole diameter, including the fine particles less than 0.2 mm. The ash content of the four wheat straw fractions varied between 7 wt % and 15 wt %. The maximum level of ash (15 wt %) was observed for the finest particle fraction based on materials less than 0.2 mm. The silicone (Si) content in corresponding ash samples of the sifted wheat straw was analysed by Energy Dispersive X-ray analysis, EDX. The silicone content increased from 18 % to 24 % at a reduced particle size. Moreover, the pH-buffering capacity of the four wheat straw fractions was reduced as the particle size of the specific fractions was increased.