Abstract: The present invention relates to an integrated process for producing diesel fuel or fuel additive from biological material by producing paraffins by a Fischer-Tropsch reaction on one hand and by a catalytic hydrodeoxygenation of bio oils and fats on the other hand. Two hydrocarbon streams, which both comprises predominately hydrocarbons of a certain chain length are treated separately and finally combined and distilled together. The invention also relates to the use of by-products of the wood-processing industry for producing diesel fuel and to a method for narrowing the chain length distribution of Fischer-Tropsch derived diesel fuel. The invention provides a high-quality middle distillate fraction from various biological sources and most preferably from by-products of the wood-processing industry.