High entropy alloys (HEAs), which comprise more than five principal elements, are presently of great interest in materials science and engineering. A predication by CALPHAD has been performed in a new AlTiNbVW HEA, which shows that this alloy consists of two similar bcc phases in the as-cast condition. Current work is to study the phase composition in this multicomponent alloy system at equiatomic condition using neutron/synchrotron diffraction under heat treatment. The chemical composition and microstructure of these two bcc phases has been determined using Energy Dispersive X-Ray Analysis (EDX) and Backscattered-Electron (BSE) Imaging. A diffusion controlled phase transformation between these two bcc phases has been found to take place between 1000°C and 1700°C. The phase transformations kinetic of this bcc1 to bcc2 transformation has been studied systematically using in-situ neutron/synchrotron diffraction with combination of a dilatometer.