Objective To investigate the role of expression of T cell costimulatory molecule CD28 and variance of T cell subpopulations in the development and prognosis of gastric cancer and colorectal cancer. Methods The peripheral blood lymphocytes were tested for T cell subpopulations and T cell costimulatory molecule CD28 by flow cytometry in 38 patients with gastric cancer, 42 patient s with colorectal cancer , and 21 healthy peoples as control group . Results Expressions of T cell costimulatory molecule CD28 in patients with gastric cancer and colorectal cancer were (25. 80 ±10. 56) % and (28. 95 ±9. 29) % , and significantly higher than that of control group 〔(0. 82 ±0. 98) % , Plt; 0. 01〕. Expression percentage of total T cell (CD3 + ) in patient s with gastric cancer and colorectal cancer were significantly lower than that of control group 〔(53. 61 ±13. 84) % and (55. 96 ±10. 68) % vs (72. 07 ±7. 83) % , Plt; 0. 01〕. Expression percentage of CD4 + T cell (CD4 + CD3 + ) in patients with gastric cancer and colorectal cancer were significantly lower than that of control group 〔( 29. 84 ±9. 71) % and ( 33. 75 ±9. 04) % vs (38. 79 ±5. 08) %; Plt; 0. 01 , Plt; 0. 05〕; Expression percentage of CTL cell (CD8 + CD28 + CD3 + ) in patient s with gastric cancer and colorectal cancer were significantly higher than that of control group 〔( 1. 57 ±1. 99) % and (1. 93 ±2. 61) % vs (0. 02 ±0. 04) %; P lt; 0. 01〕; Expression percentage of CD8 + inhibitory T cell (CD8 + CD28 -CD 3 + ) and CD4 / CD8 ratio in patient s with gastric cancer were significantly lower than that of control group 〔(16. 06 ±6. 94) % vs (20. 56 ±6. 54) % , Plt; 0. 05 ; (1. 10 ±0. 51) % vs (1. 36 ±0. 31) % , P lt; 0. 05〕; Expression of regulatory T cell (CD4 + CD25 + CD3 + ) of patients with colorectal cancer was (19. 74 ±6. 89) % , which was significantly higher than that of control group 〔(13. 72 ±3. 08) % , Plt; 0. 01〕. No difference of expression was found in peripheral T cell subpopulations of postoperative patients with gastric cancer and colorectal cancer after one week ( Pgt; 0. 05) . Conclusion T cell number is fall ,T cell costimulatory molecule CD28 useless expression is increase in patient s with gastric cancer and colorectal cancer. CD4 + T cell subpopulation is significantly decreased in patient s with gast ric cancer. The regulatory T cell of patient s with colorectal cancer is significantly increased.