We demonstrate a facile hydrothermal synthesis of SnO2/MoO3-x nanocomposites with ultrafine SnO2 crystallites uniformly embedded into an amorphous MoO3-x matrix, which demonstrate superior electrochemical performance as anodes for lithium ion batteries, including long-term cycling stability (953mAh/g after 100 cycles at 200mA/g), high rate capability (668.0mAh/g after 1000 cycles at 1000mA/g) and high initial Coulombic efficiency (81.3% at 200mA/g). Synergistic effects of the both components SnO2 and MoO3-x on the lithium storage are revealed by electrochemical characterization and supported by TEM and XPS analysis performed at the different discharge/charge states of the respective electrodes. SnO2 nanocrystallites confined within the amorphous MoO3-x matrix efficiently buffer the volume changes of Li-Sn alloying-dealloying upon cycling, while the metallic Mo in situ generated by a conversion reaction of MoO3-x promotes the reversible reaction SnO2 +Li+ ↔Sn+Li2O. In addition, the amorphous MoO3-x with bulk or surface defects allows for a better lithium insertion and thus enhanced capacity.