In the GAMMA10 tandem mirror, ion cyclotron range of frequency (ICRF) waves are effectively used for the plasma production and heating. Alfvén ion cyclotron (AIC) modes are spontaneously excited due to the strong heating. Stability boundary for the AIC modes is analyzed by introducing the effective mirror ratio. It is shown that the AIC modes become more unstable with smaller mirror ratio. The pitch angle scattering of the high energy ions due to the AIC mode are clearly observed.