Orientation-controlled Cu bicrystals with disperesed SiO₂ particles were tensile tested at various temperatures in vacuum and Ar-gas atmospheres. Two kinds of bicrystals having [001] twist 12°and 30°grain boundaries were employed. 30°bicrystal specimens fractured intergranularly at around the intermediate-temperature range, although 12°ones fractured transgranularly at all the temperatures. The elongation to fracture was smaller when deformed in Ar atmosphere than in vacuum almost at all the temperatures irrespective of the bicrystal specimens. The above results could be reasonably understood by considering the difference of the mean grain-boundary strength, the inherent grain-boundary strength, and the effect of gas atmosphere which would be responsible for the acceleration of void formation and growth.