B Background: Cancer is a disease indicated by the uncontrolled cell growth andabnormal cell spread in human tissues. The type of cancer that often occurs in the oral cavity is squamous cell carcinoma, which is a malignant tumor of stratified squamous epithelium with a percentage up to 90%. Extract lontar fruit seed coat consist of phenolic compounds, flavonoids, triterpenoids, tannins, and alkaloids. Those compounds are known to have antiproliferative, antimutagenic, anticarcinogenic, anti-inflammatory and anticancer activity. Extract lontar fruit seed coat can stop the cell cycle in the G1 / S and G2 / M phases. Therefore, the cell growth can be inhibited in cancer cells. Aim: The aim of this study was to determine the effect of ethanol extracts of lontar fruit seed coat on HSC-3 cell cytotoxicity. Method: This study conducted using experimental research and in vitro laboratory, a treatment group which divided into nine groups namely extract lontar fruit seed coat with concentration 50%, 25%, 12.5%, 6.25%, 3.125%, and 1.56%, untreated culture was a medium to used as negative controls and positive control of doxorubicin. Results: Concentration 50% of extract lontar fruit seed coat had a significant difference (p<0,05) compared to extracts at concentrations of 25%, 12.5%, 6.25%, 3.125%, 1.56%, negative control, and positive control. Conclusion: The extract of lontar fruit seed coat (Borassus flabellifer L) with a concentration of 50% was an effective concentration for increasing the cytotoxicity of HSC-3 cells. Furthermore, the inhibitory power of the lontar fruit seed coat with a concentration of 50% could outperform doxorubicin as a positive control.