Under the six times harvest in a year, the yearly and seasonal changes of their compositions, orchardgrass, tall fescue and kentucky bluegrass with clover, were compared respectively. The yearly change of the yield of grass component corresponded with the tiller density. The tiller density of orchardgrass had a tendency to decrease year after year, but that of tall fescue and kentucky bluegrass to be maintained. It owes to the different growing characters of grass species. One of the characters was the growing vitality of their upper parts during summer and autumn, and the other was the tillering capability during late autumn to early spring of next year. Tall fescue has a strong growing vitality of its upper parts in summer and autumn, and gets an advantage in competition with clover. As a result, the tiller density of tall fescue in next spring became increased than that of the spring of first year. The tiller density of orchardgrass and kentucky bluegrass with weak growing vitality in summer and autumn were strongly suppressed by competition with clover. Kentucky bluegrass recovered the tiller density during late autumn to early spring of next year than that at the spring of first year, but orchardgrass could not.