Double torsion testing was carried out to clarify the relationship between the crack growth rate and mode I stress intensity factor at the crack-tip and to investigate the characteristics of acoustic emissions associated with subcritical crack growth. Rocks used were Murata basalt and Oshima granite. Dominant frequency of acoustic emissions ranged from 100kHz to 1MHz, which obviously corresponds with the frequency range of acoustic emissions under usual uniaxial compressive loadings. Acoustic emission rate showed the close relation with the growth rate of the crack. A tendency that the dominant frequency decreased with the increase in crack velocity was observed.