Mulch film is applied to and can greatly increase rice yield in mountainous and hilly areas of Central and Southwest China. Weeds are important pests in rice fields, but little knowledge exists about how mulch film impacts weed diversity in rice fields. Herein, we investigated the composition, density, and diversity of weed communities in film-mulched and conventional rice fields and compared species richness, total and relative densities, and diversity index of weeds between the two planting patterns. Twelve and 14 species of weeds were found in the film-mulched and conventional rice fields, respectively. Of them, Portulaca oleracea, Monochoria, vaginalis, Eulaliopsis binata and Marsilea quadrifolia, which occurred in conventional rice field, did not grow in the film-mulched rice field. Total weed density was suppressed in the film-mulched field compared to conventional rice field. Digitaria sanguinalis, Juncellus serotinus, Oxalis corniculata and Cyperus difformis were the dominant weed species and their relative densities were 18.01–30.46%, 17.22–23.97%, 11.91–45.59% and 10.29–49.26%, respectively. O. corniculata and C. difformis were found in both field types, but their densities were significantly lower in the film-mulched field than in the conventional rice field. D. sanguinalis and J. serotinus were dominant species found exclusively in the film-mulched rice field. In contrast with conventional rice fields, Pielou index was higher in the film-mulched rice field. Shannon and Margalef indices did not differ between planting patterns. Film-mulching was available for controlling weeds in paddy field and may be useful for maintaining weed communities at low levels and thereby avoiding severe weed outbreaks.