Abstract：Objective：To establish a flow cytometric method for detecting minimal residual disease (MRD) in acute B lymphoblastic leukemia (B-ALL) and evaluate the prognostic significance of monitoring MRD. Methods：MRD in 33 children with definitely diagnosed B-ALL receiving SCMC-ALL-2005 protocol treatment was detected by four color fluorescence flow cytometry. All the enrolled children received follow-up visit for five years. The results of MRD detection, relapse rate and 5-year event-free survival rate were analyzed. Results：The analysis templates for the distribution of normal bone marrow cells were established. The relapse rates of 18 MRD positive and 15 MRD negative children were 55.6% (10/18) and 13.3% (2/15) respectively, and a difference with statistical significance (P＜0.05) was found. The 5-year event-free survival rates were 44% and 93% respectively in the two groups with significant difference (χ²=6.37, P＜0.05). Conclusion：The results of MRD detection showed clinical importance in prognostic assessment and may provide the reference for the selection of therapeutic protocol for children with childhood B-ALL.