Objective: Our research is to study the expression of CD 133 and ALDH 1 in CD 133+ Colo205 cells and to explore the clinical significance of ALDH1 and CD133 in the development of colorectal cancer.Methods: Colo205 cells were divided into two groups.Cells in group Ⅰ were cultured in serum-free medium (SFM) containing human recombinant EGF and bFGF and cells in group Ⅱ were exposed to serum-supplemented medium (SSM).The expression of CD133 was analyzed in both groups and cells with marker CD133+ and CD133-were obtained respectively from the SFM group using flow cytometry and their growth pattern were observed in SFM with optical microscope.The expression of CD133 and ALDH1 in CD133+ cells and CD133-cells was detected by immune fluorescence.CD133+ cells and CD133-cells were then injected subcutaneously into NOD/SCID mice and the expression of CD133 and ALDH1 was detected in tumor tissues in mice by immunohistochemistry.Result: In vitro, the percentage of CD133+ was significantly higher in SFM groups than in SSM groups (P < 0.05).CD133+ cells showed the ability to form tumorspheres in SFM, while CD133-cells failed.More over, CD133 and ALDH1 strongly expressed in CD133+ cells, but didnt expressed in CD133-cells.In vivo, when CD133+ cells and CD133-cells succeeded in tumorigenesis in vivo, CD133+ cells group positively expressed CD133 and ALDH1, but CD133-cells group failed.Conclusion: ALDH1 is one of the important cell markers for colorectal cancer stem cells.This important finding suggests both ALDH1 and CD 133 may interact together and lead to the tumorigenesis colorectal cancer.