A simple approach to the in situ deposition of well-crystallized SnO2 nanocrystals on the surface of multiwalled carbon nanotubes (MWCNTs) in the ethanol solution system of SnCl2 was presented in this paper. The well-crystallized SnO2 nanocrystals with small and uniform crystal size (4~5 nm) can be tightly and homogenously coated on the outside surface of the MWCNTs entirely. The prepared nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and nitrogen BET measurements. It is found that the hydrothermal reaction plays an important role for the crystal growth of the SnO2 nanocrystals on the surface of MWCNTs, and the crystallinity of the SnO2 nanocrystals can be controlled by varying the pH value or the hydrothermal reaction time. It is found that a unique structure of leaf-like SnO2 nanosheets hanging on the MWCNTs was obtained when using water as the solvent of SnCl2 instead of ethanol. The gas sensing performance of the SnO2/MWCNTs nanocomposites was also investigated.