Statins is HMG-CoA inhibitors which used for decreasing plasma cholesterol levels and preventing coronary artery disease. Preclinical and clinical studies showed that statin could decrease the risk of cancer. This study was performed to evaluate the cytotoxic activity of simvastatin on T47D breast cancer cell lines and its effect in cyclin D 1 expression and apoptosis. This was quasi experiment using post test with non-equivalent control group design. Simvastatin cytotoxic activity was evaluated using MTT assay. Furthermore, the effect of simvastatin on cyclin D 1 expression and apoptosis were evaluated using flow cytometry using antibody monoclonal anti-cyclin D 1 and annexin V-Pi, respectively. The results showed that simvastatin had cytotoxic activity on T47D breast cancer cell lines with an IC 50 value of 25.25 ± 1.61 µg/mL. Moreover, simvastatin in concentration range from 6.31 to 50.5 µg/mL decreased the cyclin D1 expression with an EC 50 value of 18.96±4.42 µg/mL and induced apoptosis with an EC 50 value of 26.96 ± 6.05 µg/mL. In conclusion, simvastatin inhibits T47D breast cancer cell growth through reduction of cyclin D 1 expression and induction of apoptosis. ABSTRAK Statin (HMG-CoA inhibitor) merupakan golongan obat yang digunakan untuk menurunkan kadar kolesterol plasma dan untuk mencegah jantung koroner. Penelitian praklinik dan klinik menunjukkan bahwa terapi statin dapat menurunkan risiko terjadinya kanker. Penelitian ini bertujuan mengkaji aktivitas sitotoksik simvastatin terhadap kultur sel kanker payudara T47D dan efeknya terhadap ekspresi cyclin D 1 dan apoptosis. Jenis penelitian ini adalah eksperimenal semu dengan rancangan post test with non equivalent control group. Aktivitas sitotoksi simvastatin diuji dengan metode MTT assay. Selanjutnya efek simvastatin terhadap ekspresi cyclin D 1 dan apoptosis dikaji menggunakan flowcytometry menggunakan antibody monoclonal anti-cyclin D 1 dan annexin V-Pi. Hasil penelitian menunjukkan simvastatin mempunyai aktivitas sitotoksik terhadap sel kanker payudara T47D dengan nilai IC 50 sebesar 25,25 ± 1,61 µg/mL. Simvastatin dengan kisaran konsentrasi 6,31 sampai 50,5 µg/mL mampu menurunkan ekspresi cyclin D 1 dengan nilai EC 50 sebesar 18,96 ± 4,42 µg/mL dan menginduksi apoptosis dengan nilai EC 50 sebesar 26,96 ± 6,05 µg/mL. Sebagai kesimpulan, simvastatin menghambat pertumbuhan sel kanker payudara T47D dengan menurunkan ekspresi cyclin D 1 dan menginduksi apoptosis.