Oriented strandboards (OSB) having various properties were constructed by varying processing parameters including strand thickness, strand length, and panel density. A calculation method was developed for analyzing the fractal dimension of void size (FDVS) on the cross-section of OSB samples based on a computer image processing technique and the fractal geometry theory. The results showed that The FDVS on the cross-section of OSB varied with different processing parameters. The FDVS decreased with strand thickness and increased with panel density, whereas the FDVS irregularly changed with strand length. Especially for panels with the same overall porosity, the FDVS was dependent on the internal structure. Therefore, the FDVS could be a useful additional parameter for characterizing the internal structure of OSB.