Poplar is a fast-growing, short-rotation species that has become a feedstock for pulping and for different composite wood products, such as fiberboard and particleboard. Because of the short rotation, unique characteristics, such as high content of juvenile wood, are present. The objective of this research was to investigate the effects that incorporating poplar in wood chips has on the resultant fiber quality and on fiber-refining energy consumption. Two different quantities of Chinese poplar (Populus lasiocarpa Oliv.) chips were separately mixed into wood chips consisting of a mixture of larch (Larix Mill.) and Masson pine (Pinus massoniana Lamb.). The mixed wood chips went through the refining process in a local medium-density fiberboard mill and refining parameters/conditions, energy consumption, and fiber size were examined at the production site. The results showed that the incorporation of poplar played a favorable role in terms of the fiber size and energy consumption. The higher the poplar content, the better the fiber quality and the lower the energy consumption during refining.