Objective To discuss the clinical characteristics,blood amino acid and urinary organic acid changes,the relationship between clinical phenotype and genotype in methylmalonic acidemia(MMA). Method The clinical data,blood amino acid,urinary organic acid and gene results of the 10 patients diagnosed with MMA were analyzed,which were from the developmental pediatrics department of Shengjing Hospital of China Medical University during August 2011 to December 2012. Results 10 patients,age of onset ranged from 5 days to 2 years and 5 months.The main clinical manifestations included poor feeding (2 cases),poor weight gain(1 case),repeated vomiting (2 cases),repeated convulsions (1 case),developmental retardation (3 cases),developmental degradation (1 case) and musclar dystonia (3 cases).The laboratry findings showed metabolic acidosis in three cases,hyperammonemia in seven cases and hyperhomocysteinemia in six cases.The typical MRI changes were found in 3 cases,nonspecific change were found in 4 cases.4 cases were detected related gene mutation,diagnosed three cases of cblC subtype,one case of mut-subtype.In patients reactive to VitB12 treatment,urinary MMA average decreased rate was 6.45,which of the blood C3 was 2.05,and C3/C2 was 1.64. Conclusions Except for the nonspecific clinical features,it is vulnerable to have metabolic acidosis,hyperammonemia and hyperhomocysteinemia; the brain MRI appears both typical changes and non-specific changes;the decline level of urinary MMA is a sensitive index in patients reactive to VitB12 treatment;genetic testing is used for clinical parting,estimating prognosis and prenatal diagnosis.