Knowledge of the actual Nb3Sn filaments' thermal strain, epsilon(th), in a cable-in-conduit conductor (CICC) is essential to predict the CICC performance in operation starting from the strand scaling laws for the critical current density J(c)(B, T, epsilon). To obtain a measurement of epsilon(th) under relevant conditions, i.e. at low temperature and with the mechanical constraints of a long length section of CICC, the critical temperature as a function of the applied field, T-c(B), is measured by an inductive method for the CICC in situ and for the freestanding filaments used for the cable manufacture. To deduce the thermal strain in the CICC, the T-c(B) results are compared with the T-c(B, epsilon) curve. Starting from the susceptibility curves measured for both the CICC and the filaments, it is possible to compute the T-c distribution in the CICC using a deconvolution algorithm. The first results of T-c measured on two CICCs in the SULTAN test facility suggest a broad distribution of thermal strain, peaked at negative strain, which remains almost constant during the cyclic loading. From the knowledge of both the thermal strain distribution and the actual CICC performances, it will be possible to discriminate between reversible and irreversible degradation in Nb3Sn CICC.