In situ TEM observations of defects and. the amorphous phase in Si wafer during 150 keV Ar+ ion implantation were made which elucidated their characteristic behavior in Si. Defects introduced by ion implantation were eliminated by amorphous phase formation and then new defects did not form in the amorphous phase. Microstructural evolution in Si wafers under high dose implantation (2E16 ions/cm2) of 400 keV Si* ions was also investigated at temperatures of -70, -30, 20, 100 and 200 °C using a cross-section 1 TEM observation technique. At temperature of 20 °C and above, a defect layer was formed in each specimen, and the defect density was observed to decrease as temperature increased. At temperture of -30 °C and below the amorphous phase was formed and a defect layer which made contact with this phase was also observed. After annealing of these implanted specimens at 850 °C for 20 min, the amorphous phase had crystallized and the defect layer in contact with the amorphous phase was almost eliminated. But another defect layer was formed during annealing in the region where the amorphous phase had existed.