Diamond crystals nucleate in the centers of spherical concentric-shell graphitic particles where high pressure prevails under electron irradiation at specimen temperatures above 900 K. The diamond crystals grow under further irradiation until the graphitic particles have wholly transformed to diamond. Hence, the conversion of the graphitic structure to diamond starts at high pressure and proceeds at decreasing, possibly even at zero, pressure. The experiment is carried out in a transmission electron microscope which enables us to monitor this phase transformationin situon an atomic scale. It is shown that the graphite/diamond system under electron irradiation is highly dissipative. This suggests that the phase transformation at low pressure occurs via self-organization under nonequilibrium conditions. ©1997 American Institute of Physics.