Background:  Cell viability is an important factor in radiation therapy and thus is a method to quantify the effect of the therapy. Materials and Methods: The viability of human hepatoma (HepG2) cells exposed to radiation was evaluated by both the MTT and Trypan blue assays. The cells were seeded on 96 well-plates at a density of 1 x 10⁴ cells/well, incubated overnight, and irradiated with 1-100 Gy. Results: The cell viability was decreased in a dose- and time- dependent manner when using the Trypan blue assay, but no significant changes in the response to dose could be detected using the MTT assay. It indicated that the MTT assay was not efficient at a cell density of 1 x 10⁴ cells/well on 96 well-plates to determine cell viability. Subsequently, the relationship between cell viability and lower cell density (1 x 10³, 3 x 10³, and 5 x 10³ cells/well) was investigated. A cell density of 1 x 10³ was found to be the most effective when using the MTT assay. Results show that the cell density is most important when using the MTT assay in 96 well-plates to follow in radiation effects. Furthermore, the radiation-induced cell viability dependent on cell density was confirmed by using the traditional Clonogenic assay. Conclusion: Our results suggest that the MTT and Trypan blue assays are rapid methods to detect radiation-induced cell viability of HepG2 cells in about 3 days as compared with 14 days of assay time in the Clonogenic assay. To obtain accurate cell viability measures using both rapid assays, an incubation time of at least 3 days is needed after irradiation.