Series of new pyrazole derivatives have been successfully synthesized, their purity confirmed by thin-layer chromatography and the chemical structures identified by some spectroscopic techniques like 1H-NMR, APT 13C-NMR and FT-IR. Two synthetic precursors were synthesized: The first compound was 5-Fluoro-2,3,3-trimethyli-3H-indole (1) and the second compound was 2-(5-Fluoro-3,3-idimethyl-1,3dihydro-indoli-2-ylidene)-malonaldehyde (2). The target compounds were obtained from the reaction of compound (2) with different substituted phenylhydrazine hydrochloride. The toxicity of the new synthesized compounds was tested against breast cancer cell lines (MCF-7) and results showed that some cancer cells were killed.