Abstract Thermal expansion of Bi2212/Ag round wire was measured using double extensometers between 290 and 5 K. The Cu–Be extensometers with gauge length of 50 mm were directly attached to the sample and change of sample length during both cooling and warming processes was evaluated. Irreversible behavior was found in thermal expansion of the composite wire as a function of temperature measured during cooling and warming processes. On the other hand, in the second warming, no such clear irreversibility was observed compared with the curve during the first warming. Thermal expansion of Bi2212 filaments was estimated from the elastic region of measured data of the composite wire by stress analysis based on the rule of mixture. To fit the measured data, smaller work-hardening coefficient of Ag was obtained in the 1st cooling than those in the following processes. It was concluded that irreversibility of thermal expansion of the composite wire is attributed to the narrow elastic region and small work-hardening coefficient of the Ag component during 1st cooling.