Recent observations of instabilities in MHz frequency range in theNational Spherical Torus experiments (NSTX) and their analysis arereported. The instability frequency is correlated with the characteristicAlfvén velocity of the plasma, vA. The observed instabilitiesare explained as compressional Alfvén eigenmodes (CAEs) drivenby energetic beam ions. Based on a localized CAE solution, mode frequenciesare essentially the compressional Alfvén frequency at the modelocation and are given by ωCAE = vAm/r, where mis the poloidal mode number, and r is the minor radius. Energeticions that are in Doppler shifted cyclotron resonance with CAEs providefree energy to destabilize CAEs. Properties of CAE instability drivenby different neutral-beam injection ion distributions are presented.