Coarse woody debris (CWD) play important structural and biogeochemical roles in forest ecosystem processes. The aim of this study was to determine the wood density and biomass of beech and hornbeam and its relationship with different degrees of decay and diameter classes in Khairud educational and research forest, Nowshahr city, Mazandaran province. First, in an area of 50 hectares, CWD (with a median diameter of more than 7.5 cm) were fully survived and inventoried. Then, to determine the wood density of CWD, three cubic samples (close to the pith, close to the bark, and between the pith and bark) with dimensions of 2 × 2 × 2 cm were taken from each CWD and transferred to the laboratory. After determining the types of wood density (wet, dry, and critical density) in the laboratory, the samples were grinded, and using standard laboratory methods, the carbon and nitrogen elements of CWD were measured based on different decay grades and diameter classes. The results showed that the wood density types decrease with an increasing degree of CWD decay grade. Also, the dry wood density of Beech CWD and downed Beech CWD decreases significantly with increasing diameter. The results related to the amount of biomass showed that this value increases with an increasing degree of decay. Based on diameter classes, with an increasing diameter of CWD, the amount of biomass showed an increasing trend. The results also showed that with increasing degree of decay and diameter of CWD, their carbon storage increases. But in the case of nitrogen storage, due to the lack of significant differences, no specific pattern was observed. In general, it can be said that the presence of large CWD in the forest is very important ecologically, because stores more carbon. On the other hand, the preservation of CWD in the forest is in line with close to nature management.