Heartwood in tree stems provides natural durability and ornamental values to wood products as well as antimicrobial properties to protect living stems from decay. Control of the amount and quality of heartwood is required because of its importance in wood utilization. Despite the importance, the mechanism of heartwood formation has been poorly understood. To obtain further knowledge for understanding this mechanism, we studied the seasonal variation of various components of heartwood formation in Larix kaempferi (Lamb.) Carrière. Dehydration of tracheids at the boundary between sapwood and intermediate wood occurred continuously during the study period. The death of ray parenchyma cells at intermediate wood occurred from spring to summer. The deposition of heartwood substance took place in autumn to winter. Thus, different components of heartwood formation were active in different seasons. Since heartwood formation is a system consisting of several components within a series of continuing processes, understanding the seasonal change of heartwood formation requires monitoring these components simultaneously.