The isomer formation of three isomers of C78 fullerene is discussed on the basis of experimental evidence revealed by pressure-controlled fullerene production. Theoretically predicted most stable isomer, C2v’-C78 has been found to be very sensitive to the graphite burning condition. The fraction of C2v’ isomer dramatically changes from zero to 50% by changing the foreign gas pressure. The present findings strongly suggest that an isomerization within the C78 carbon network such as a pyracylene rearrangement type transformation unlikely takes place.