This study was purposed to investigate the clinical efficiencies and adverse reactions of treating the myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) by using decitabine. The clinical data of 12 MDS and AML patients treated with decitabine were analyzed retrospectively. Among 12 patients there were 1 case of MDS-RA, 2 cases of MDS-RAEB-I, 3 cases of MDS-RAEB-II, 2 cases of AML-M4, 2 cases of AML-M5, 1 case of AML-M6 and 1 case of AML-M0. In decitabine chemotherapy program for 5 days (n = 8), decitabine 20 mg/(m(2)·d) × 5 days was applied, 4 weeks for 1 cycle; in program for 3 days (n = 2), decitabine 15 mg/m(2), once 8 h for 3 days, 6 weeks for 1 cycle; another program (n = 2), decitabine 20 mg/(m(2)·d) every other day for 5 times. For 1 patient achieved complete remission (CR) after treatment with decitabine, ID4 gene methylated level was detected by MS-PCR and ML-PCR before and after treatment. The results showed that 2 cases achieved CR, 1 case partial remission, 5 cases stable disease, 1 case progress of disease and 3 cases died. Disease control rate was 66.67% (8/12), the effective rate 25% (3/12). The average survival time was (11.5 ± 2.1) months. 1-year OS rate was 40%, 2-year OS rate was 16.7%. MS-PCR detection showed that the decitabine could significantly reduce the ID4 gene methylation level. It is concluded that decitabine can stabilize disease status of MDS patients, reduce blood transfusion dependence and improve the life quality of patients, and even some patients who transformed from MDS to leukemia achieved CR after treatment with decitabine. Decitabine can reduce the ID4 gene methylation level. The main adverse reaction of decitabine was myelosuppression, infection and so on. So the blood transfusions, antibiotics and other supportive treatments for these patients are needed. Most of patients well tolerate the adverse effects of decitabine after active symptomatic and supportive treatment. The efficacy and survival rate of patients in this study were similar to that of application of decitabine to treat MDS in other domestic studies.