Abstract Background Hepatocellular carcinoma (HCC) is one of the most malignant tumor types and has a high incidence and mortality. Many miRNAs play important roles in the development of HCC. Identification of these miRNAs and their targets is increasingly urgent for a better understandingof miRNA function in both physiological and pathological contexts. Many studies have shown that the expression of let-7 is often downregulated in the process of tumorigenesis, suggesting that let-7 may participate in this process as an oncogene. Methods Immunochemistry staining was used to observe the expression of let-7b in HCC tissues. A CCK-8 assay was employed to detect the role of let-7b in the proliferation of HCC cells. The cell cycle of HCC cells was examined by flow cytometry. BALB/c nu/nu mice were used to detect the tumorigenesis potential of HCC cells; western blot and real-time PCR were employed to observe the expression of p21 in HCC cells. Results In our previous studies investigating HCC tissue samples obtained from the national tissue samples bank of liver cancer in Eastern Hepatobiliary Surgery Hospital, we found one abnormal expression of miRNA (let-7b), which was significantly downregulated in HCC tissue. In the current work, we studied the relationship between let-7b and HCC to potentially provide invaluable information for developing novel therapeutic strategies for treating HCC. Based on our findings, let-7b expression was absent in HCC tumors, and its lower expression was associated with poor prognosis of HCC. In further experiments, we found that let-7b inhibited HCC cell proliferation through upregulation of p21. Conclusion The results of our study suggested that let-7b might inhibit the proliferation of HCC cells by upregulating p21.