Electron irradiation of thin (4-6 monolayers) nanocrystals of C-60 fullerenes formed on Si(111)-(7x7) substrates were performed with intense flux of field-emission (FE) electrons (10(5) -10(8) electrons nm(-2) s(-1)) extracted at 10-100 V from probe tips of a scanning tunneling microscope (STM). At extraction voltage of 10-30 V, stimulated migration of C-60 caused long-lasting changes of morphology of nanocrystals on a time scale of minutes after irradiation. With increase of extraction voltage beyond 40 V, fragmentation of C-60 became dominant leading to creation of large carbon structures with apparent height of 0.4-1.5 nm as a result of coalescence of fragments. The contributions of different phenomena including polymerization and evaporation to nanocrystal degradation vary with electron energy and dose.