Introduction: Phyllanthus species such as P. urinaria, P. niruri and P. debilis are common herbs found in Malaysia that are traditionally used for treatment of chronic diseases such as liver diseases, kidney stones and cancer. Phyllanthus species were shown in vitro to have many biological functions such as anti-cancer, anti-inflammatory, hepatoprotective and anti-diabetic. Aims: The goal of this in vitro study was to assess the cytotoxic effect of the methanolic extract of P. urinaria, P. niruri and P. debilis on MCF-10A and MCF-7 cells (i.e., normal and cancerous breast cell lines) and to determine the therapeutic index of each Phyllanthus species. Materials and Methods: We determined the therapeutic index for each Phyllanthus sp. and its selective toxicity towards these cells. The toxicity of sample toward the cells are measured by trypan blue cell counting method. Results: Our results showed that P. debilis had the lowest IC50 concentration in MCF-7 cells and the highest IC50 concentration in MCF-10A cells and its therapeutic index was higher than that found in P. niruri and P. urinaria. The high therapeutic index of P. debilis suggests that this species has greater selective cytotoxicity in MCF-7 cancer cells than in MCF-10A normal cells. Thus, the methanolic extract of P. debilis should be further characterised and developed for future use as an anti-cancer agent.