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We studied the selective formation of Co catalyst particles as a function of indentation pressure. We subjected a Co (8 nm thickness)/Si substrate pre-annealed at 600 °C to indentation processing. The catalytic function was confirmed in the indentations by the selective growth of carbon nanotubes (CNTs) at 800 °C. The number density of CNTs against the indentation pressure was investigated against indentation loads for two types of indenter: a Berkovich indenter with a ridge angle of 115° and a Berkovich indenter with a ridge angle of 90°. The pressures above 7 GPa applied by the former indenter enhanced Co atomization acting as a catalyst function for CNT growth (35 CNTs in one indentation). In contrast to this, the number of CNTs was markedly reduced when the latter indenter was used with pressures less than 3 GPa. The pop-out phenomenon was observed in unloading curves at pressures above 7 GPa. These results indicate that metastable Si promotes the self-aggregation of catalyst particles (Co) leading to the selective growth of CNTs within indentations at pressures above 7 GPa.