Libraries of spiro[cyclopropane-1,3'-indolin]-2'-ones were synthesized and evaluated for their biological activity against five different human cancer cell lines HT-29 (colon cancer), DU-145 (prostate cancer), Hela (cervical cancer), A-549 (Lung cancer), and MCF-7 (breast cancer). Many compounds of the series exhibited promising anticancer activity (IC50<20 μM) against the studied cell lines. Based on the screening results, a structure activity relationship (SAR) of the pharmacophore was proposed. Among the series compound 6b and 6u showed significant activity against human prostate cancer cell line, DU-145. Flow cytometric analysis showed that these two compounds arrested the cell cycle in the G0/G1 phase leading to caspase-3 dependent apoptotic cell death. Further, measurement of mitochondrial membrane potential and Annexin V-FITC assay also suggested that 6b and 6u induced cell death by apoptosis.