To computationally address the structural complexities of high entropy alloys (HEAs), we have applied atomistic modeling methods to predict the stable solid-solution phases in a series of AlxCoCrFeNi (x= 0 to 2.5) HEAs. In our atomistic simulations, the interatomic interactions were described with the modified embedded atom method (MEAM). Using the Monte Carlo (MC) simulations based on the developed MEAM potentials, we sampled the thermodynamically equilibrium structures of the AlxCoCrFeNi HEAs containing varying Al content at 1300 K. Considering both configurational and vibrational entropy contributions, we predicted that the AlxCoCrFeNi HEAs would form a single fcc solid-solution phase when x<0.44, a single bcc solid solution phase when x>1.75, and mixed fcc-bcc phases with x in between. Our theoretical results are quite consistent with experimental observations. In addition, we found that some Al-Ni rich B2-type long-range ordered phases could also exist in the bcc solid solution of the AlxCoCrFeNi (x>1.0) alloys.