In this study, the alkaloid compounds of Solanum nigrum have been evaluated. The chemical compositions of the leaf methanol extract of S. nigrum were investigated using gas chromatography-mass specroscopy (GC-MS). GC-MS analysis of S. nigrum alkaloid leaf methanol extract revealed the existence of the cyclopentasiloxane-decamethyl, L-proline, ethylester, 2-ethyl-1-butanol, methyl ether, cyclopentasiloxane-ocamethyl, betanedioic acid, hydroxyl, diethyl, ester, 1.1.3.3.5.5.7.7-octamethy-l-7-(2methyl-propoxy) tetrasiloxane-1, dodecanoic acid, 3-hydroxy-, ethyl ester, cyclopentasiloxane-ocamethyl, dodecanedoic acid, bistert-butyldimethylsilyl ester, 2-pyrrolidinecarboxylic acid-5-oxo, ethyl ester, 1-dodecanamine, N.N-dimethyl, cyclooctasiloxane, hexadecamethyl, 5-keto-2, 2-dimethylheptanoic acid, ethyl ester, cyclodecasiloxane, eicosamethyl, 9.12.15-octadecatrienoic acid, octadecanoic acid, octadecenal, 9-octadecenamide, octadecane, 3-ethyl-5-(ethylbutyl), N-acetyl-L-tryptophan ethyl ester, ethyl iso-allocholate, phthalic acid, di(2-propylpentyl)ester and 17-(1.5-Dimethylhexyl)-10. 13-dimethyl-2.3.4.7.8.9.10.11. 12.13.14.15.16.17-tetradecahydro-1H. Alkaloids extract from leaf of S. nigrum were assayed for in vitro antibacterial activity against Escherichia coli, Proteus mirabilis, Staphylococcus aureus, Pseudomonas aerogenosa and Klebsiella pneumonia using the diffusion method in agar. The zone of inhibition was compared with different standard antibiotics. The diameters of inhibition zones ranged from 0.8 to 2.01 mm for all treatments.