The MDM2 proto-oncogene is overexpressed in many human tumors. Although MDM2 inhibits tumor-suppressor function of p53, there exists a p53-independent role for MDM2 in tumorigenesis. Therefore, downregulation of MDM2 has been considered an attractive therapeutic strategy. Hispolon extracted from Phellinus species was found to induce epidermoid and gastric cancer cell apoptosis. However, the mechanisms are not fully understood. Herein, we report our findings that hispolon inhibited breast and bladder cancer cell growth, regardless of p53 status. Furthermore, p21(WAF1), a cyclin-dependent kinase inhibitor, was elevated in hispolon-treated cells. MDM2, a negative regulator of p21(WAF1), was ubiquitinated and degraded after hispolon treatment. We also found that activated ERK1/2 (extracellular signal-regulated kinase1/2) was recruited to MDM2 and involved in mediating MDM2 ubiquitination. Based on this finding, we investigated whether the sensitivity of cells to hispolon was related to ERK1/2 activity. The results indicated that cells with higher ERK1/2 activity were more sensitive to hispolon. In addition, hispolon-induced caspase-7 cleavage was inhibited by the ERK1/2 inhibitor, U0126. In conclusion, hispolon ubiquitinates and downregulates MDM2 via MDM2-recruited activated ERK1/2. Therefore, hispolon may be a potential anti-tumor agent in breast and bladder cancers.