Defects in Pb and Ge nanocrystals deposited on thinned Si (110) substrates have been studied by a 200-kV ultrahigh vacuum field emission transmission electron microscope (UHV-FE-TEM). The nanocrystals ranged from 2 to 10 nm in size and contained several types of defects that are observable in the [110] direction. High-resolution transmission electron microscopy (HRTEM) indicated that the most frequently observed defects in these particles are stacking faults, twins and Frank partial dislocations. The possibility of defect formation in Pb nanocrystals is much higher than that. in Ge nanocrystals. Frank partial dislocations are detected both near the surface and at the central part of Ge nanoparticles. The structural fluctuation of Pb nanocrystals has been recorded at a video rate of 1/60 s, showing the movement and annihilation of the defects. In Pb nanocrystals, Frank partial dislocations and twin boundaries (TBs) moved within 1 s, while the defects in Ge nanocrystals were stationary.