Background: The tumor microenvironment has been shown to affect the responsiveness of immunotherapy. Effective anti-tumor immune response requires the activation and expansion of specific antigen-reactive T cell clones. It was reported that increased T cell clonality was associated with improved response to immunotherapy. However, what type of tumor microenvironment facilitates T cell clonal expansion remained controversial. The study aims to investigate the correlation between tumor microenvironment and the clonality of the T cell repertoire in lung cancer and colorectal cancer. Methods: 4 lung cancer patients and 4 colorectal cancer patients were enrolled in this study. Tumor tissues and peripheral blood samples were collected for RNA sequencing and T cell receptor CDR3 sequencing. The infiltration levels of 28 immune cells were estimated based on the mRNA expression of the genetic markers. The T cell clonality was defined as 1-Peilou’s evenness. Data were presented as mean± S.E.M. Results: The mean T cell clone counts in the blood samples of the 8 patients were 25676±4782 (ranging from 10259 to 45016), and the mean clonality of the TCR repertoires was 0.20±0.02 (ranging from 0.11 to 0.27). The clonality of T cells in colorectal cancer patients was similar to that of the lung cancer patients (0.22±0.02 versus 0.18±0.03, p = 0.31), showing comparable potentials of antigenic responses. The tumor infiltration of regulatory T cells, type 17 T helper cells, CD56^bright natural killer cells, and natural killer cells varied greatly among patients, the coefficient of variation of those cells were 54.61%, 54.61%, 54.43%, and 55.62% respectively. In contrast, the coefficient of variation of monocytes was 23.34%, displaying a relatively even distribution among patients. The Pearson’s correlation coefficient was calculated to show the correlation between T cell clonality and the infiltration level of all 28 types of immune cells. Notably, only the infiltration of type 17 T helper cells significantly associated with T cell clonality, the positive correlation gave an R square value of 0.68 (r = 0.82, 95% confidence interval of 0.04-0.98, p = 0.04). The infiltration levels of CD4+ T cells, CD8+ T cells, regulatory T cells, type 1 and type 2 T cells, and gamma delta T cells were not affected by T cell clonal expansion. The expression of B cells, dendritic cells, macrophages, natural killer cells, and monocytes did not correlate with T cell clonal expansion. However, the abundance of neutrophils appears to positively correlate with T cell clonality (p = 0.09). Conclusions: The clonal expansion was significantly associated with the infiltration of type 17 T helper cells but not other subtypes of T cells, showing that the type 17 T helper cells are crucial to the antigenic responses in lung cancer and colorectal cancer. A neutrophil enriched tumor microenvironment may facilitate T cell clonal expansion.