Decomposition pattern of two contrasting green manures viz, water hyacinth (Eichhomia crassipes) and mimosa (Mimosa invisa) was studied in an incubation experiment at the department of Agricultural Chemistry, Bangladesh Agricultural University, Mymensingh, during the period from May to July, 2003. The soil was amended with the manures @ 1 g 50 g-1 soil. A basal dose of 250 pg N, 200 pg P and 250 pg K g-1 soil was also applied to each experimental unit. Microbial respiration as a measure of decomposition was monitored over 50 days at room temperature and different intervals of time. Of the amendments, mimosa decomposed more rapidly (14.3% vs. 3.8% by day 2) than water hyacinth. The highest decomposition rate was observed in mimosa at day 2 and water hyacinth at day 10. The decomposition of mimosa was approximately two times higher than water hyacinth. After 50 days of incubation, a total of 32 and 49% of the added C were respired from water hyacinth and mimosa, respectively. Both the rate and total decomposition of the green manures were also found to be related to their nutrient status (C : N : P : S ratios).