Two melting transitions were observed in linear segmented polyurethane-urea elastomers underextension using thermal, mechanical and X-ray diffraction techniques, and the results are compared.These data indicate both strain-induced and temperature-induced crystallization in the stretchedclastomers, which may result from two different types of crystallites with different melting tempera-tures. These have been assigned as type 1 appearing around 60℃, and type 2 around 30℃. Thetype 1 crystallization can be induced by stretching at room temperature to large strain, and is mechani-cally reversible, but the type 2 crystallization is mainly induced by cooling below its crystallizationtemperature. These two crystalline structures are interchangeable under suitable conditions. Atelongations greater than 300%, the low temperature peak observed on fusion thermograms disappearsor combines with the high temperature peak. When the temperature of the sample is over the meltingpoint of the type 1 crystal, irreversible melting occurs and only the type 2 crystal develops on cooling.The results of stress-strain and stress hysteresis experiments at different temperatures indicate therelative importance of strain-induced and temperature-induced crystallization on the mechanicalproperties of these materials. Two melting transitions were observed in linear segmented polyurethane-urea elastomers underextension using thermal, mechanical and X-ray diffraction techniques, and the results are compared. The data are both data-induced and temperature-induced crystallization in the stretchedclastomers, which may result from These two types of crystallites with different melting tempera-tures. These have been assigned as type 1 appearing around 60 ° C, and type 2 around 30 ° C. The type 1 crystallization can be induced by stretching at room temperature to large strains, and is mechani- cally reversible, but the type 2 crystallization is mainly induced by cooling below its crystallization temperature. When these two crystalline structures are interchangeable under suitable conditions. Atelongations greater than 300%, the low temperature peak observed on fusion thermograms disappearsor combines with the high temperature peak. When the temperature of the sample is over the melting point of the type 1 c rystal, irreversible melting occurs and only the type 2 crystal develops on cooling. the results of stress-strain and stress hysteresis experiments at different temperatures indicate therelative importance of strain-induced and temperature-induced crystallization on the mechanical properties of these materials.