Phenolic compounds exhibit several health protective properties. Galangin, curcumin, pycnogenol, puerarin and ursolic acid are commonly used plant phenolics in folk medicine. The aim of our study was to evaluate the difference between neutral red uptake (NRU) and MTT assays using different plant phenolics (galangin, curcumin, pycnogenol, puerarin and ursolic acid) in healthy and cancer cells in different time periods. In this study, the cytotoxic effects of these phenolic compounds were investigated by NRU and MTT assays in healthy (V79, Chinese hamster fibroblast cell line) and cancer human cervix epithelial adenocarcinoma cell line Henrietta Lacks (HeLa) and human mammary carcinoma cell line (BT-474) in 18, 24 and 48 h incubation periods. Our results demonstrated that galangin, curcumin, pycnogenol, puerarin and ursolic acid decreased cell viability of V79, HeLa and BT-474 cells in a dose-dependent manner in 18, 24 and 48 h incubation periods. However, the cell survival rate was much lower in 48 h incubation period. There was no difference between the results from NRU and MTT assays. To decide which incubation period and which cytotoxicity study to be used, the cytotoxicity mechanism of the compound must be known.