Both heat-treated PVA fibre and the untreated one are alike degradated oxidatively by aqueous potassium permanganate. The degradation is proportioned to the concentration and temperature of KMnO4 solution, but is small in the acidic oxidation. In the degradation, Carboxyl-groups and Carbonyl-groups are produced at the ends of molecules, and the amount of the former is larger than that of the latter. In the acidic oxidation, the amount of the Carbonyl-groups increases. In the main chain also, the carbonyl-groups are produced and its total amount is nearly twice as large as that of the degradation. In proportion to the concentration and temperature of KMnO4, the strength and elongation decreases, and the denier increases. In the heat-treated fibres, compared with the untreated ones, degradation is great, but the decrease of the strength and elongation is small.(Received November 16, 1970)