We report experimentally determined self-diffusion coefficients for carbon in a type-IaA diamond at pressure and temperature conditions within diamond stability field at 10 GPa and 2075-2375 K. The activation energy of diffusion is 6.8 +/- 1.6 eV and a preexponent constant is of 4.1 x 10(-5) m(2)/s [ln(-10.1 +/- 2.0) m(2)/s]. The activation energy is approximately 30% lower than results predicted previously from ab initio calculations.