The nucleation of diamond on the (100) and (110) edge atoms of the basal plane of graphite has been investigated theoretically by using ah initio molecular orbital theory, including the effects of electron correlation by means of second-order Moller-Plesset (MP2) perturbation theory. Outgrowths of partially saturated carbon ring systems (acting as precursors for diamond formation) from both the (100) and (110) edge atoms are calculated to be energetically more favorable than the corresponding growth of graphitic carbon ring systems by about 250 and 175 kJ/mol (per added C atom), respectively. The extra hydrogens in the partially saturated outgrowths from the (100) edge atoms are shown to stabilize the buckled carbon ring, formed during the diamond nucleation process, by about 391 kJ/mol. This is an average value obtained when two H atoms are simultaneously added to two neighbouring C radical atoms. The probability for diamond nucleation was concluded to be larger for the zigzag edge (100) than for the armchair edge (110) of the graphitic (001) plane.