Gastric cancer is a serious public health cancer and causes nearly 1 million deaths a year worldwide. Th1 cells play critical roles in orchestrating the adaptive immune responses against gastric cancer. T-bet, a member of the T-box family of transcription factors, is the Th1 master regulator and up-regulated during Th1 differentiation. Polymorphisms have also been shown to exist in T-bet. Some reports indicated that some tumours were associated with the drift of Th1 and Th2. In the present work, we investigated the drift of Th1/Th2 by detecting the expression levels of T-bet, IFN-γ, IL-4, and GATA-3 in peripheral blood mononuclear cell of gastric cancer patients by real-time PCR, explored the relationship between the polymorphism of T-bet gene and drift of Th1/Th2 by gene sequence, western blot, and gene transfection. Our results indicated that a predominant Th2 phenotype was existence. T-bet gene mutations may be associated with Th2-dominated condition in gastric cancers.