Weeds are major competitors of rice growing in the farming land. Supply of nutrients for the productivity of rice enhances weed growth. Identification of major weeds from agricultural land and their quantitative estimation as competitors must be required for proper management of weeds. Present study was conducted in tubs and plastic trays to identify the growth status of weeds compared with the growth record of rice (in vitro study) and estimate the biochemical activities during competition.Commelina diffusa andAlternanthera sessilis are the common perennial weeds found in all rice fields of West Bengal, India. Different morphological parameters of rice plants and weeds were estimated after seven days of intervals for three months. Biochemical study was also conducted after four week of plantation of rice plants and weeds. Growth of rice was inhibited due to profuge growth of Commelina diffusa andAlternanthera sessilis. Yield of grains and plant biomass were also decreased. This study revealed that Alternanthera sessilis was more competitor of rice than Commelina diffusa. Without proper management of weeds, maximum rice productivity would not have been possible.