Flavan-3-ols consist of flavan-3-ol monomers and polymers with different degrees of polymerization (DP). In this study, flavan-3-ol extracts from grape seeds were well separated into three fractions including monomers, oligomers (2 < DP < 10) and polymers (DP > 10), by means of normal-phase HPLC-MS. The different patterns of these three fractions were analyzed in three Vitis vinifera cultivars ('Shiraz', 'Cabernet Sauvignon' and 'Marselan') seeds from veraison to harvest. The results showed: (1) polymers were the main form of flavan-3-ols in grape seeds and monomers accounted for only a small proportion; (2) the contents of flavan-3-ol monomers in the seeds of three grape cultivars all exhibited a gradually decreasing trend with a little fluctuation, whereas the patterns of the change of contents of oligomers and polymers were extremely different among grape cultivars; the contents of flavan-3-ol oligomers were enhanced in the seeds of 'Cabernet Sauvignon', but were reduced in the other two cultivars; (3) with regard to the proportion of flavan-3-ols with a certain DP to total flavan-3-ols, both flavan-3-ol monomers and flavan-3-ols with low DP fell in proportion, while the flavan-3-ols with high DP increased correspondingly. These findings indicate that flavan-3-ol polymerization in developing seeds is variety-dependent and may be genetically regulated.