Observations of the flowering phenology and measurements of the heights of understory herbaceous plants were made in a cool temperate deciduous forest, where light availability is relatively low and fluctuates markedly through the year, and it is too cold for many plants to grow in winter. Ninety-one species were recorded between April and October. The number of flowering species as a function of date showed a bimodal distribution. The plants flowering in spring and those flowering from late summer to early autumn each accounted for about 40% of the number of species. The plants that flowered in spring were smaller than those that flowered later in the season. The springflowering plants would produce flower buds in the previous growing season, as the time from the appearance of the aerial part to flowering was transient. Some species completed their main life history events during the spring as spring ephemerals. The small sizes in these species may result from bud formation in the previous 8eason and/or the short period of growth. The plants that flowered in late summer and early autumn, by contrast, were large. These plants should have relatively long periods of vegetative growth and flower at the end of the growing season using matter produced in that year. A long vegetative growth period would tend to make plants firm and/or large. It was suggested that flowering phenology was separated into two periods by the climatic and environmental constraints of a cool temperate deciduous forest.Plant flowering phenology has a close relationship with life history strategy and is affected by various factors including climatic conditions, habitat environments, and interactions with other organisms (Rathcke and Lacey 1985). In a given biome, the flowering phenology pattern of a plant community through the year shows trends that reflect the constraints of climatic factors and habitat conditions; many herbaceous plants flower immediately after the wet season in South India (Sivaraj and Krishnamurthy 1989), dry conditions caused by low river levels trigger flowering in East African riverine forests (Kinnaird 1992), flowering phenology is constrained by a cool and short growing season in the Arctic (Thbhallsd6ttir 1998), and so on.The light regime in a habitat is considered to affect the production of matter in plants. On a forest floor, light availability is relatively low. Especially in a cool temperate deciduous forest, light availability fluctuates markedly through the year and many plants are unable to grow in winter. Uemura (1994) tested leaf phenology patterns in a forest understory and suggested that leaf habits have relationships with the light regime of habitats and climatic conditions. Because reproduction is limited by matter production, the flowering pattern of the understory herb community may be also related to climatic and environmental conditions. However, only a few studies have focussed on the flowering phenology of the understory herbaceous community in a temperate deciduous forest in terms of habitat condi...