The upstream process of comminution is a key element in the use of renewable raw materials, which impacts the consecutive disintegration of the materials. Energy efficiency of the comminuition process is therefore of utmost importance. The key factors to increase energy efficiency are, beside the mill type and the mill operation factors, the species of the renewable resource, in terms of water contend and the mechanical properties which are the dominant factors in biomass size reduction. In this study the influence of different factors on the effective specific comminution energy (ESCE) is elucidated. For theses purpose, three types of raw wood chips as well as recycled wood of two different qualities, were comminuted with a cutting mill and a swing hammer mill. The materials were comminuted at several levels of moister contend, under varied opening sizes of the internal screen of the mills. Particle size distributions of the comminuted materials were examined with sieve analyses and dynamic image analysis. Especially, the moister content directly influences the ESCE and the particle size distribution. Moreover the type of material, due to its different mechanical properties, is of significant influence on the particle size distribution and the ESCE.