The Nb3Sn superconducting strands used for ITER (international thermal-nuclear experimental reactor) consist of brittle intermetallic compounds. Due to the different coefficients of thermal expansion and different moduli of elasticity among the components, thermal strain is generated in the composite from heat treatment temperature down to room temperature, even to operating temperature (4.2 or 4.5 K). In order to research thermal strain and build the mathematic model of the Nb3Sn strands, local strain measurements during heating by means of quantum beams and tensile tests at room temperature were carried out. According to the elasto-plastic behavior of Nb3Sn revealed in the stress curves, the thermal strain was evaluated. The results show that the local strain exerted on superconducting (SC) filaments along the axial direction is compressive at room temperature and tensile at high temperatures. Moreover, the temperature dependence of thermal strain can be reproduced well by the numerical calculation method proposed in the paper.