A field experiment was conducted during the kharif seasons of 2005 and 2006 at the research farm of Uttar Banga Krishi Viswavidyalaya with the objectives to study weed dynamics and yield reduction due to weedcrop competition as well as to determine critical period of crop-weed competition in transplanted and wet seeded rainfed rice. Ten treatments comprising weedy condition for first 20 days after transplanting (DAT)/15 days after sowing (DAS) then weed free, first 40 DAT/30 DAS then weed free, first 60 DAT/45 DAS then weed free, first 80 DAT/60 DAS then weed free, weed free condition for first 20 DAT/15 DAS then weedy, first 40 DAT/30 DAS then weedy, first 60 DAT/45 DAS then weedy, first 80 DAT/60 DAS then weedy, season long weed free and complete weedy conditions were tested in randomized block design with three replications for transplanted/wet seeded rice, respectively. Among the weeds appeared in transplanted rice, the broadleaved weeds Monochoria hastata, Ludwigia parviflora and Nymphoides indicum and grass weed Echinochloa crusgalli were dominant and aggressive because of their long emergence profile. In wet seeded rice field, the broadleaved weeds Monochoria hastata, Ludwigia perennis and the sedges Cyperus flavidus, Cyperus difformis, Fimbristylis miliacea, Scirpus juncoides and grass Cynodon dactylon were aggressive and continuously emerged throughout the crop growth. Critical period of weed-crop competition in transplanted and wet seeded rice was from 20 to 40 DAT and 15 to 60 DAS, respectively. Weedy situation throughout the crop growth caused yield reduction to the tune of 57 to 61% in case of transplanted rice and 64 to 66% in case of wet seeded rice in comparison to season long weed free situation