Abstract: To understand the invading capability of alligator weed (Alternanthera philoxeroide) in cultivated rice fields and rice resistance to A. philoxeroide invasion, the growth characteristics of A. philoxeroide and effects on rice development at early invasion were studied. A paddy field invasion simulation was conducted through transplanting propagators of A. philoxeroide into cultivated rice fields after seedling transplant. The results showed that A. philoxeroide growth weakened in the first cultivation season of rice after field transplanting. The numbers of clones and erect caulis of A. philoxeroide decreased slightly from early tillering to late tillering and also increased slightly from late tillering to mature stage of rice (all insignificant). Although weed height increased significantly, weed stem grew small and soft with weak tops growth. A. philoxeroide weed leaves were generally small, weak and curly and produced few new buds in rice fields. No significant effects were noted regarding rice growth parameters such as number of tillers, effective tillering per cluster and plant height when only few A. philoxeroides propagators were transplanted into rice fields. A. philoxeroide near rice field margins and in rice field levees hardly grew or spread into main rice fields. Although only weed clone number, erect caulis and plant height increased significantly, weed vegetation cover grew thick. These findings suggested that both cultivated lands and rice fields controlled growth and reproduction of A. philoxeroides. It was also proposed to keep rice fields under cultivation to prevent weed invasion.