<p>Background: Recently, due to the steep increase in cancer lethality statistics, pharmaceutical societies seek approaches for designing drugs with higher efficiency and lower expenses. Plant-based drugs have therefore gained much attention, due to their abundance and ease of accessibility, and their higher effectiveness.&nbsp;<br /> Methods: Wild-grown caper [<em>Capparis spinosa (C. spinosa)</em>] was collected from northern Iran and next 100 <em>g</em> of the powder was added to 300<em> ml</em> of a solvent (Ethanol 80), the solution was mixed for 72 <em>hr</em> and later filtered <em>via</em> Whatman filter papers. The solvent was taken out under vacuum conditions and extracts were then collected and stored in glass vials. The High Pressure Liquid Chromatography (HPLC) method was used to assay quercetin which consisted of the following specifications: C18 column, UV detector wavelength of 260 <em>nm</em>, mobile phase acetonitrile and water and flow rate of 1 <em>ml/min</em>. In this study, the anti-cancer effects of <em>C. spinosa</em> extract on HeLa, MCF7, Saos and Fibroblast cancer cell lines have been investigated.<br /> Results: The amount of quercetin was assessed by HPLC. The anti-tumor activity and the antioxidant level of hydroalcoholic extract of <em>C. spinosa</em> have been evaluated with MTT assay and FRAP technique, respectively. HPLC data showed quercetin form the major component of <em>C. spinosa</em> extract. In addition, FRAP data indicated that <em>C. spinosa</em> extract had high antioxidant activity and MTT assay indicated that <em>C. spinosa</em> extract effectively decreased the cancer cell lines.<br /> Conclusion: The quercetin in <em>C. spinosa </em>extract had significant anti-tumor effects and may be regarded as an ideal natural drug for cancer therapy.</p>