Developmental processes and biomass allocation of a multi-stemmed shrub, Stephanandra incisa, were measured to reveal the advantages of a multi-stemmed growth form on matter production. Stephanandra incisa is a low shrub species that mainly grows in the forest understory. The current-year shoots become shorter by yearly branching that effectively increases the allocation ratio to photosynthetic organs. The frequency distribution of relative current-year shoot length (the ratio of current-year shoot length to stem length) in each ramet (a stem and its crown) of a clump (genet) were skewed to larger length classes in younger ramets and gradually skewed to shorter length classes in older ramets. The biomass allocation ratio of non-photosynthetic organs to photosynthetic organs in ramets changed from a declining trend to a rising trend as the age of the ramets increased. The biomass allocation ratios of ramets and clumps increased with their size, but the ratio of clumps showed lower values than that of ramets of the same size. The age and size structures of ramets in clumps contributed to the narrow variability of the biomass allocation ratio. Stephanandra incisa showed a high biomass allocation ratio to leaves and maintained the ratio with a multi-stemmed growth form.