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A series polycrystalline samples of YBa2Cu3-xZnxO7-δ (x = 0.05, 0.10, 0.15, 0.20, and 0.25, respectively) were studied by means of differential scanning calorimetry, thermogravimetry, X-ray diffraction, and resistance-temperature measurement. It is found that there are two unknown subtle phase transitions in each compound before melting. At about 430oC and 800oC the samples show an endothermal anomaly and a weight loss, respectively. The amount of Zn-dopant affects the phase transitions obviously. Careful analysis of the experimental results demonstrates that two-phase transitions come out of different structural blocks, perovskite and rock salt, in the compounds.