The present study is to evaluate micro-density of timber materials on cell level. Density, one of the most important parameters for such materials, is discussed in details. Four kinds of timbers are placed under a microscope for cell photos from which the cavity ratios are obtained through an appropriate image processing computer code. Micro density is calculated by dividing bulk density by cell wall ratio. It is found that three out of four kinds of timbers have similar micro-density and therefore, for those samples, macro-density can be expressed as the multiplication of the cell wall ratio and micro-density which is treated as one of their characteristic constants, no matter it is late wood or early wood. In the future, micro-density can be one of the criteria to categorize structural timbers.