〔Abstract〕 Objective To explore the changes of frequency, phenotype and function of peripheral blood mucosal-associated invariant T(MAIT) cells in colorectal cancer patients compared with healthy controls. Methods Peripheral blood mononuclear cells(PBMCs) were isolated from human peripheral blood, which were collected from the healthy donors and colorectal cancer patients, with density gradient centrifugation, and the cells were activated by PMA + ionomycin. The frequency, phenotype and intracellular cytokine production of MAIT cells were determined by flow cytometry. Results ① The frequency and absolute number of peripheral blood MAIT cells with colorectal cancer patients were lower than those in healthy controls(P=0.022,P<0.001).② In the same patient, the frequency and absolute number of peripheral blood MAIT cells before and after chemotherapy were no difference(P=0.507,P=0.743). ③ Compared with healthy controls, the levels of PD-1 and CD69 molecules on peripheral blood MAIT cells surface in colorectal cancer patients were both different(P=0.049,P<0.001), while Tim-3 molecule level was not different(P=0.757). ④ There was no difference in the function of MAIT cells in patients with colorectal cancer compared with healthy controls(P=0.545,P=0.121). ⑤ The frequency and absolute number of MAIT cells were not significantly correlated with clinical tumor stage(P=0.261,P=0.173). Conclusion Compared with healthy controls, the frequency and absolute number of MAIT cells from peripheral blood of colorectal cancer patients decrease, the phenotype change, but the function is no difference. Chemotherapy has no effect on the frequency of MAIT cells in the peripheral blood of colorectal cancer patients. The frequency of MAIT cells is not correlated with clinical tumor stage as well.