We report results of thermal expansion (TE) and magnetostriction (MS) measurements on a single crystal sample of Gd₅(Si_0.5Ge_0.5)₄ prepared by the Bridgman method. TE and MS were measured along the c axis by the strain gauge method and the temperature was controlled using a closed cycle helium refrigerator. From the TE measurements the magnetic structural phase transition temperature was found to be 259.5 K on cooling and 261.5 K on heating. The abrupt change in strain and the temperature hysteresis indicate that it is a first order transition. MS measurements were conducted at 15, 258, and 265 K. At 15 K, the magnetostriction amplitude was 3–4 ppm, whereas at 258 K it was 100 ppm. At 265 K, which is just above the Curie temperature, a giant magnetostriction of 2000 ppm was found. This unusual behavior is due to the fact that the external magnetic field can increase the transition temperature above 265 K, resulting in a first order magnetic/structural phase transition. The results reveal that giant magnetostriction inGd₅(Si_0.5Ge_0.5)₄ only occurs as a result of the magnetic/structural transformation.