Quercetin appears to have many benefical effects on human health, including cardiovascular protection, anti-cancer activity, anti-ulcer effects, anti-allergy activity, cataract prevention, anti-viral activity and anti- inflammatory effects. Tamoxifen is a non-steroidal anti-estrogen drug widely used in the treatment of patients with estrogen receptor-positive breast cancer. We have compared the effects of quercetin, which has not been used as a drug, on apoptosis and telomerase enzyme activity in NIH-3T3 cell lines with the effects of tamoxifen. Apoptosis was determined by TUNEL method and telomerase enzyme activity was determined by ELISA method. In NIH-3T3 cell lines, it was determined that 100 mM of quercetin induced significantly apoptosis after 24 and 72 hours when compared with that after 48hours. With other doses no significant changes were detected. Quercetin also decreased telomerase enzyme activities in NIH-3T3 cell lines after 48 and 72 hours when compared to control. Only 100 mM of tamoxifen induced apoptosis after 72 hours comparable to that obtained with quercetin. It was determined that all tamoxifen doses decreased telomerase enzyme acitivity such as doses of quercetin. We showed that, quercetin has effects like tamoxifen, but it induces significantly apoptosis when compared with tamoxifen. In conclusion, flavonoids such as quercetin, have benefical effects on cancer therapy. Effects such as reduced telomerase enzyme activity and induced apoptosis must be studied much more to assist in developing new therapeutic pathways. There should be much more studies in order to discover quercetin and other potential medicines. We hope that these findings may be helpful in preventing and treating cancer.