Fibers coated with polymer are prepared by adding the polymer to a fiber material, typically cellulose pulp, and subjecting the fiber material with the added polymer to impact of grinding forces in a defibrator, typically a hammer mill. The resulting fibers are very efficiently coated with the polymer and are suitable as binder fibers in fibrous materials consisting completely or partially of the coated fibers bonded through the polymer, and as reinforcing fibers in fiber-reinforced cement products. The polymer is preferably applied as a hot melt, and the defibration is performed immediately after the application.