Abstract: Objective: To observe the changes in C- myc mRNA expression in HCT- 8 colorectal carcinoma cells treated with phenylacetate (PA). Methods: HCT- 8 cells were cultured in the presence of PA at 1.0, 2.0, 3.0, 4.0, and 5.0 mmol/L. The cellular proliferation inhibitory ratio was evaluated by MTT assay 24h, 48h, and 72h later, and C- myc mRNA expression in the HCT- 8 cell line was detected by an in situ hybridization test. Results: With increased PA concentration and extended culture time, the inhibitory rate of tumor cell growth was notably increased. The inhibitory rates were 5.1%～ 24.3%,16.7%～ 72.3%, and 30.2%～ 93.4% in cells cultured for 24h, 48h, and 72h, respectively. The expression rate of C- myc was lower in cells cultured in PA than that in cells cultured without PA, with a significant difference (P<0.05). Conclusion: PA can downregulate C- myc mRNA expression and inhibit the growth of colorectal carcinoma cells.