We investigated the damage of silicon strip sensors due to heavy-ion radiation as a check of the in-orbit stability of silicon strip sensors under cosmic-ray irradiation in the five-year gamma-ray large-area space telescope (GLAST) mission. In order to study single-event effects (SEE), we used Fe ions slowed-down in an absorber, with a resulting linear energy transfer (LET) of 8 MeV/(mg/cm(2)) on the surface of the sensor. The total doses achieved in two runs were about 8 krd and 22 krd, corresponding to a fluence of about 5 x 10(7) and 1.5 x 10(8) ions/cm(2), respectively. Silicon strip sensor with two different crystal orientations <111> and <100> were irradiated. We measured leakage currents and capacitances before and after irradiation to evaluate the damage. The leakage current density was found to increase by about 10 (nA/cm(2))/krd, as. expected for the ionizing irradiation. No significant changes of capacitances were found. In addition, no coupling capacitors were broken. The observed effects are well within the in-orbit requirements of the GLAST mission.