The identification of phytochemical compounds is based on the peak area, retention time molecular weight, molecular formula, MS Fragment- ions and Pharmacological actions. GC-MS analysis of Cassia fistula revealed the presence of the Oxacyclododecan-2-one, Imidazole,2-amino-5-[(2-carboxy)vinyl], D-Glucose, 6-O-?-D-galactopyranosyl, 2-Nonanone, Eicosanoic acid, phenylmethyl ester, Phenol, 4-(2-propenyl), Eugenol, Caryophyllene, ?-copaene, Azulene,1,2,3,3a,4,5,6,7-octahydro-1,4-dimethyl-7-(1-methylethenyl), ?-acorenol, Spiro[5.5] undec-8-en-1-one, Isoaromadendrene epoxide, Tetraacetyl-d-xylonic nitrile, Benzyl Benzoate, N-Isobutyl-(2E,4Z,8Z,10E)-dodecatetraenamide, Phenethylamine, 3-benzyloxy-2-fluoro-?-hydroxy, 4a-Hydroxy-4-nitroperhydronaphthalen-1-one, Dasycarpidan -1-methanol, acetate (ester), Propanoic acid, 2-(3-acetoxy-4,4,14-trimethylandrost-8-en-17-yl), Carda-4,20(22)-dienolide,3-[(6-deoxy-3-O-methyl-?-L-mannopyranosyl, Cis-13-Eicosenoic acid, 16-Nitrobicyclo[10.4.0]hexadecane-1-ol-13-one, Strychane,1-acetyl-20?-hydroxy-16-methylene, 2,4,6-Decatrienoic acid, 1a,2,5,5a,6,9,10,10a-octahydro-5,5a-dihydro, Vitamin E and Glycine,N-[(3?,5?,12?)-3,12-dihydroxy-24-oxocholan-24-yl]. The FTIR analysis of Cassia fistula leaves proved the presence of Alkenes, Aliphatic fluoro compounds, Alcohols, Ethers, Carboxlic acids, Esters, Nitro Compounds, Alkanes, Alcohols and Phenol. Cassia fistula was highly active against Aspergillus terreus (6.99±0.29). Methanolic extract of bioactive compounds of Cassia fistula was assayed for in vitro antibacterial activity against eleven pathogenic bacteria by using the diffusion method in agar. The zone of inhibition were compared with different standard antibiotics. The diameters of inhibition zones ranged from 1.00±0.05 to 6.02±0.23 mm for all treatments.