A series of (3,5-dimethyl-1H-pyrazol-4-yl)-phenyl-diazenes were prepared by condensing hydrazine hydrate with 3-phenylazo-pentane-2, 4 dione. The synthesized compounds were screened for their antibacterial activity against four types of bacteria. The (3,5-dimethyl-1Hpyrazol- 4-yl)-(4-chloro-phenyl)-diazene showed strong activity against E.coli, B.cereus, P.vulgaris selectively.