Phosphorylation on certain Ser/Thr-Pro motifs is a major oncogenic mechanism. The conformation and function of phosphorylated Ser/Thr-Pro motifs are further regulated by the prolyl isomerase Pin1. Pin1 has been shown to be prevalently overexpressed in human breast cancer cell lines and cancer tissues and to play a critical role in the transformation of mammary epithelial cells by activating multiple oncogenic pathways. Pin1 expression was found to be an excellent independent prognostic marker in prostate cancer. However, little is known about Pin1 and its downstream targets beta-catenin and cyclin D1 expressions in human oral cancers. In the present study, we quantified Pin1 expression in 74 paired normal/tumor human oral cancer samples as well as oral cancer cell lines. Pin1 was overexpressed in oral squamous cell carcinoma (OSCC) and its level correlated with beta-catenin accumulation and cyclin D1 expression. Moreover, we examined Pin1 mRNA expression in OSCC and cancer cell lines by RT-PCR analysis. The results showed that there is concordance in the relationship between the Pin1 mRNA level and Pin1 protein expression. The up-regulation of Pin1 mRNA expression in tumor part when comparing with that in non-tumor part was in agreement with that of the Pin1 protein overexpressed in OSCC. In addition, we showed that the molecular and immunohistochemical profiles of Pin1 overexpression were associated with progression of OSCC. Taken together, these results indicate that Pin1 is a regulator of cyclin D1 expression in OSCC and might play a role in oral oncogenesis. The overexpression of Pin1 can be used as an indicator for pathological diagnosis of OSCC.