We have studied the temperature and magnetic field dependence of the magnetic hysteresis of YBa2Cu3Oy single crystals with different oxygen contents. Different behaviors in the hysteresis loop were observed between oxygenated and oxygen-deficient crystals. Although the hysteresis loop of the latter exhibits a scaling feature, the second peak in the former collapses by fast relaxation of the magnetization. AM-T curve of the former crystal was found at a certain temperature Tj just below the irreversibility line. The value of Tj in magnetic field H obeys an expression H α(Tc - Tj)^2. This magnetization jump indicates a discontinuous change in vortex density. These results are discussed on the basis of the first-order vortex-lattice melting transition.
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