[en] In this paper, we report unusual thermal hysteresis behaviour in half-doped manganites Nd_0.5Sr_0.5-xCa_xMnO₃ (0.0 ≤ x ≤ 0.50). For 0.05 ≤ x < 0.25 samples, the magnetization curves show a counterclockwise hysteresis loop. For 0.25 < x ≤ 0.40 samples, however, they exhibit a clockwise loop. The inverse hysteretic behaviour stems from the competition between two kinds of strain, namely, the compressive strain and the tensile strain. The former produces a counterclockwise thermal hysteresis while the latter leads to a clockwise one. In the meantime, a magnetic instability is observed and the strain relaxation should be the main reason for it