The nature of stacking faults and partial dislocations in deformed Ni3Ga single crystal having the L12 structure has been analysed by transmission electron microscopy on the basis of the diffraction theory of the electron microscope images. The results showed that stacking faults are of the superlattice intrinsic type and that the majority of stacking fault ribbons are bounded by two superlattice Shockley partials with parallel and opposite a/3 Burgers vectors. This latter fact is incompatible with any formation mechanisms so far proposed. Most partials lay parallel to directions 30deg apart from the Burgers vector due probably to the sessile dissociation mechansim proposed by Giamei et al. The formation mechanism of stacking faults is discussed.