The utilization of fly-ash in concrete as partial replacement of cement is gaining immense importance today, mainly on account of the improvement in the long term durability of concrete combined with ecological benefits. Technological improvements in thermal power plant operations and fly-ash collection systems have resulted in improving the consistency of fly-ash. The present study was carried out to determine the effect of high volume fly ash on concrete mechanical properties. Different types of concrete mixes were prepared that consist of 40%, 50%, 60% and 70% of class F fly ash by weight of cement which where compared with plain concrete. The mechanical properties of high volume fly ash concrete are evaluated for 7, 28, and 56 days.