An environmentally benign, simple and mild approach has been adopted for the biogenic fabrication of Ag, Cu and Au MNPs (Metal nanoparticles) using Syzygium cumini leaf extract, at room temperature. This is an aqueous phase synthesis and excludes the use of toxic organic solvents. The TEM images revealed that the as-prepared MNPs possess spherical morphology with an average diameter of 6nm. Application of these MNPs as nanocatalyst was investigated in decomposition of two organic dyes CR (Congo red) and RB (Rose bengal) in the presence of sodium borohydride (NaBH4), which is otherwise extremely slow. The rate constants of decomposition reactions were determined and found to follow the orderk(Ag)>k(Cu)>>k(Au)>k(uncat), for both the dyes. Difference in the catalytic activity arise from the difference in their work function values. High catalytic activity of Ag NPs, as compared to Cu and Au, is attributed to its least work function value. [GRAPHICS] .