Micro-structural development of TC21 alloy after double alpha/beta solution and aging heat treatment was investigated. Primary alpha morphology was determined by the first solution treatment at the top of alpha/beta portion. Varying the cooling rates can significantly influence the volume fraction of retained beta phase and secondary alpha. Higher cooling rate resulted in greater retention of the beta phase and transformation of the phase into secondary alpha phase upon aging. This higher amount of fine, secondary alpha phase promoted higher strength and lower toughness. Second alpha/beta heat treatment temperature significantly influenced the heat treatment response of the microstructure. Higher heat treatment temperatures promoted greater amounts of retained beta with fine, transformed alpha in the final microstructures. Lower alpha/beta heat treatment temperatures promoted less responsive retained beta during aging process. Aging treatment promoted decomposition of retained beta phase, particularly in larger retained-beta regions that exhibited lower stability.