Breast cancer is the leading cause of cancer deaths among females in Malaysia. Ocimum tenuiflorum L., (O. tenuiflorum) commonly known as ruku in Malaysia, is usually cultivated as a garden ornamental plant because of its small purplish and some yellowish flower. The specific objective of this research is to investigate the anticancer of O. tenuiflorum against human breast cancer cell lines (MCF-7 and MDA-MB-231) and human fibroblast cell line (HS-27). In addition, another objective is to determine the mineral and heavy metal determination of O. tenuiflorum. O. tenuiflorum exhibited anticancer activity against MCF-7 (a hormone-dependent breast cancer cell line). The viability of MCF-7 cells decreased significantly after treatment with various concentrations of methanolic plant extracts (25 and 100 µg/mL), as shown via 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyltetrazolium bromide (MTT) assay. The crude extracts show the lower IC 50 (less than 100 µg/mL) value against the cancer cell lines and show no effect on HS-27. The high content of calcium in the leaves of O. tenuiflorum may play a role in decreasing the risk of certain cancer. The concentrations of heavy metals (Pb and As) detected in O. tenuiflorum are safe for consumption.