The present invention relates to a modified fiber product and a method of producing same. According to the method, the cellulosic raw material is formed into a fiber suspension, into which components modifying the properties of the fibers are added, subsequent to which the fibrous material is dried in connection with, for example, web forming. According to the invention, alkyl derivative of cellulose is mixed into the fiber suspension in alkaline conditions, the derivative being at least partly dissolved in water, and the derivative is allowed to be sorbed into the fibrous material prior to drying such that the sorbed cellulose derivative can not be washed off with water. The strength properties or the water retention of paper and paperboard products can be improved by using the invention.