Microstructure evolutions of an as-cast AlCoCrFeNiTi0.5 high-entropy alloy after heat treatments 72 h at 500-1100℃ were investigated. The alloy shows a dendritic structure which dendrite is a spinodal structure composed of (Cr,Fe)-rich BCC phase and (Al,Ni)-rich ordered BCC phase, and the interdendrite comprises (Cr,Fe)-rich BCC, Heusler-type (Ni,Al,Ti)-rich β΄, and σ phases. After heat treatments, the BCC phase transforms into σ phase in interdendrite at 600℃, the shape of BCC phase in dendrite changes from plate to particle at 800℃, and the spinodal structure vanishes in dendrite replaced by an ordered BCC phase at 1000℃; meanwhile, both β΄ and σ phases in interdendrire transform into a BCC phase at 1000℃. Furthermore, profuse DO3 nanoparticles with 20 nm in size precipitate in BCC phase at 1000℃. These nanoparticles coarsen to 50 nm at 1100℃ and parts of them grow into 150 nm squares which are rich in Cr, Fe, Co, and Ni.