Abstract: Three steroidal saponinses, collettinside Ⅱ, Ⅲ and Ⅳ, which were isolated from the rhizomes of Dioscorea collettii Hook. f., were studied by means of FAB-MIKES mass spectrometry. Their structures areas follows: 3-O-α-L-rhamnopyranosyl (1→4)-β-D-glucopyranosyl-yamogeninⅡ; 3-O-α-L-rhamnopyranosy (1→4)-α-L-rhamnopyranosyl (1→2)-β-D-glucopyranosyl -yamogeninⅢ; 3-O-β-D-glucopyranosyl (1→3)-α-L-rhamnopyranosyl (1→2)-β-D-glucopyranosyl-yamogeninⅣ. The molecular weight of the sample can be obtained in FAB spectra and the information of sequences of the sugar groups can be determinated with MIKES,