Cuprous oxide (Cu2O) is naturally p-type due to a high concentration of Cu vacancies. N-type doping of Cu2O by diffusing Cu atoms into Cu2O is proposed and demonstrated. It is suggested that Cu diffusion in Cu2O annihilates Cu vacancies, leading to Cu interstitials and/or O vacancies which are responsible for the n-type conductivity. The effect of Cu diffusion on structural and electrical properties of Cu2O is investigated in the temperature range of 100-400°C. The resistivity of Cu-doped Cu2O sharply decreases at 350°C and the lowest resistivity of 28 Ω-cm is obtained at annealing temperature of 400°C. A diffusion coefficient of 1.91×10-16 m 2/sec is extracted at 325°C. Photocurrent measurements confirm that Cu-doped Cu2O is indeed n-type.