The invention relates to a method for defibering recycled fiber material in a drum pulper provided with a substantially horizontal drum. Recycled fiber material and water are fed to the drum pulper and the drum of the drum pulper is simultaneously rotated. When the drum rotates, the recycled fiber material rises up in the rotating drum and finally, due to gravitational force, falls back down. After falling and hitting to the bottom, the recycled fiber material is defibered, forming a fiber pulp mixture with water. A value of at least one parameter describing the defibration process or fiber pulp properties is measured on the basis of the drum pulper or the resultant fiber pulp mixture, and this value is used for controlling the rotation speed of the drum of the drum pulper. The invention also relates to a drum pulper implementing the method of the invention.