A series of novel 3-(1H-indole-3-yl)-1H-pyrazole-5-carbohydrazide derivatives 4Ia-n, 4IIa-b and 6 were prepared by hydrazinolysis of ethyl 3-(1H-indole-3-yl)-1H-pyrazole-5-carboxylate with hydrazine hydrate in excellent yields. These new compounds were fully characterized by spectroscopic methods, and the important intermediates 3Ie, 3IIc and 3IId were further confirmed by X-ray crystallography. All the new compounds were evaluated for their cytotoxic activity against 4 human cancer cell lines by MTT method. Some of them exhibited more potent antiproliferative activity against HepG-2, BGC823 and BT474 cell lines than the positive drug 5-fluorourcail. Flow cytometry analysis showed that 4Ik and 4Il arrested the cell cycle at S phase.