Abstract: Biological characteristics of Sagittaria trifolia Linn.were observed in the fields of the late-season rice.Their effects on the rice yield were evaluated.Most S.trifolia seedlings sprouted 30 days after the late-season rice fields were plowed and leveled.The sprouting might last for 46 days.The numbers of green leaves and total leaves on the plant provided a quantifiable index for the weed growth.They increased,stabilized and decreased during the weed's lifespan,while the number of withered leaves always increased with age.On average,0.5-0.8 new leaves appeared every 3 days.The average length and width of the leaves ranged from 8.45 to 15.14cm and 1.23 to 2.59cm,respectively.The average stem length reached 40.53 cm,and the average number of roots came to 44.73.Branching was not found in S.trifolia.The durations of bloom and fruit were 28-day and 12-day,respectively;and the average number of flowers and fruits were 2.15 and 1.85,respectively.About 70% of the corms of S.trifolia were found in 5 to 15 cm beneath the surface of soil.The rice yield could be reduced by 2.94% to 23.53% depending on the growth density of the weed.When the number of S.trifolia was 20 to 25 per square meter,the weeds started to induce a significant on rice production.