Characterization of carbohydrate structures using mass spectrometry is a challenging task. Understanding the dissociation mechanisms of carbohydrates in the gas phase is crucial for characterizing these structures through tandem mass spectrometry. In this study, we investigated the collision-induced dissociation (CID) of glucose, galactose, and mannose in their linear forms, as well as the linear forms of hexose at the reducing end of 1-6 linked disaccharides, using quantum chemistry calculations and tandem mass spectrometry. Our results suggest that the dehydration reaction in linear structures is unlikely to occur due to the significantly high reaction barrier compared to those of C═O migration and C-C bond cleavage. We demonstrate that the different intensities of the cross-ring fragments observed in the CID spectra can be explained by the different transition state energies of C═O migration and C2-C3, C3-C4, and C4-C5 bond cleavages, and the branching ratios of the cross-ring fragments are significantly different between glucose and galactose. The application of the cross-ring fragments to oligosaccharides reveals that the stereoisomers of glucose and galactose in oligosaccharides can be differentiated based on the relative intensities of the cross-ring fragments produced by the C2-C3 bond cleavage and C3-C4 bond cleavage, a differentiation that cannot be achieved by conventional tandem mass spectrometry.