The differential thermal analysis (DSC) with slow scanning rate (SSDSC used as abbreviation for slow scanning DSC) can play an important role for the precise determination of phase diagram.The importance and notifications for SSDSC is demonstrated with three alloy systems.Expanded peaks and thermal effects from unequilibrium reactions during DSC process can be clarified by applying SSDSC.The combination of in-situ annealing with SSDSC is shown to be useful for separating thermal effects within small temperature interval.Combination with metallographical investigation can further improve the understanding of real phase relations and exclude the unequilibrium thermal effects.