It is well known that loss of the cyclin-dependent kinase inhibitor p27Kip1 protein correlates with the poor prognosis of various cancers including oral squamous cell carcinoma (SCC). Posttranslational degradation of p27Kip1 protein is mediated by phosphorylation of Thr-187 of p27Kip1 protein, which follows ubiquitination. In this study, we constructed an expression vector expressing mutant type p27Kip1 gene (pcDNA3.1-p27Kip1 mt), with mutation of Thr-187/Pro-188 (ACGCCC) to Met-187/Ile-188 (ATGATC), which is not influenced by ubiquitin-mediated degradation. We transfected mutant and wild type p27Kip1 genes into an oral SCC cell line, B88 to up-regulate the expression of mutant or wild p27Kip1 gene in each transfectant. B88-p27Kip1 mt showed significant growth inhibition than B88-p27Kip1 wt or B88-neo in vitro (p < 0.01). Also, both types of B88-p27Kip1 showed G1 arrest and apoptosis, however, B88-p27Kip1 mt showed remarkable G1 arrest. In addition, B88-p27Kip1 mt and B88-p27Kip1 wt showed markedly inhibition of the migration and out-growth of cancer cells than B88-p27Kip1 wt or B88-neo. Moreover, B88-p27Kip1 mt also showed remarkable suppression of tumor growth and cervical lymph metastasis than B88-p27Kip1 wt or B88-neo in vivo (p < 0.01). In short, the mutant type p27Kip1 gene could show more potent antitumor effects than wild type p27Kip1 gene in B88 cells. These findings suggest that mutant type p27Kip1 gene has the potential to become a novel and powerful gene therapy tool for patients with oral cancers.