Positron lifetime experiments under optical illumination have been performed for n-type 6H SiC irradiated with 2 MeV electrons (dose: 1x10(17) e(-)/cm(2)). The positron lifetime increases after irradiation indicating the presence of vacancy-type defects. We found that the positron lifetime distinctly decreases upon illumination from that in the darkness below I I OK. After the annealing above 1300 degreesC the illumination effect tends to disappear with the disappearance of vacancy-type defects. The illumination effect appeared with a threshold photon energy of 0.47 eV. It was further found that the illumination effect exhibits in a time of 10 min after switching off the light at 15 K.