Abstract:  Objective To analyze the clinical features and gene mutation in mthylmalonic acidemia (MMA) accompanied by homocysteinemia (cblC), and review the relevant literatures. Methods The clinical features of 3 cases of MMA diagnosed by gene detection were retrospectively analyzed, and meanwhile the pertinent literatures of pathogenesis of MMA, especially combined with late-onset cblC and its gene detection, were reviewed. Results Patient 1 (26 days old) suffered from intermittent convulsions for 3 days, with isosuccinic acid 175.8 μmol/L, C3/ C2 rate 1.363, homocysteine >?65 μmol/L and abnormal EEG. MMACHC gene detection found an exon deficiency (delEXON1), which has not been reported. Patient 2 ( 12 year old) was hospitalized for limb shaking, hyperspasmia and vomiting. His isosuccinic acid level was 334.3 μmol/L, C3/?C2 rate was 0.37, homocysteine >?65 μmol/L, and had abnormal EEG. MMACHC gene detection found the mutations of c.482G?>?A and c.609G?>?A. Patient 3 was hospitalized for intermittent convulsions for 20 days, whose isosuccinic acid, C3/?C2 rate, and homocysteine were increased. MMACHC gene detection found the mutations of c.394C?>?T and c.540del8 and c.540del8 had not been reported. Review of literatures discovered that MMA was combined with epileptic seizure in some patents, which further validate that the mutation in MMACHC gene c.482G?>?A may be related to the late-onset of cblC. Conclusions Gene detection contributes to the diagnosis of MMA; the mutation of MMACHC gene c.482G>A may be related to the late-onset of cblC; delEXON1 and c.540del8 are new mutations which have not been reported.