The objective of this study was to transfer benefits of three-layered particleboards to medium density fiberboard (MDF) manufacture by using coarse fibers and, thus use less energy and lower-cost fibers as core layer material. In the first phase of this study, the effect of wood fiber size in the core layer on the properties of MDF was investigated. In the second phase, the effect of surface to core layer ratio (30/70, 40/60, 50/50, 60/40, and 70/30) on the properties of the MDF was investigated. The surface layers of the panels consisted of fine fibers. The wood fibers were produced using a thermo-mechanical refining process. The length and thickness of the fibers considerably increased with increasing defibrator discs distance. The 24-h TS values of the MDF specimens decreased from 36.8 to 34.2% as the fiber length in the core layer was increased from 4.3 to 11.5 mm. However, further increases in the fiber length increased TS values. Similarly, the bending strength, bending modulus, and internal bond strength increased with increasing fiber length (up to 11.5 mm) and thickness (up to 0.73 mm). The bending properties of the MDF specimens improved with increasing surface layer ratio, while the internal bond strength decreased.