Thermal behavior of incompatible PS/HDPE blonds is characterized by DSC thermogram. Crystallinity and heat of fusion of the blend are increased with the HDPE content. Overlapping of peaks by PS glass transition and HDPE melting prevents from using the onset point as characteristic temperature. Instead, the deflection point (temperture where the second derivative of energy flow against temperature becomes zero ) after the onset and the peak temperture are used to examine the thermal behavior of PS/HDPE blends. The deflection temperature is linearly increased with the HDPE content, However, the peak temperature shown a mild maximum at PS/HDPE=30/70, which may be attributable to the phase morphology of the blend. That is, at a given heating rate of l0℃/min, particle size and spacing of the dispersed phase (PS), and the low heat capacity of HDPE medium could be balanced to yield a maximum at PS/ HDPE=30/70. When one neglects the variation of peak temperature for the high HDPE content (70% or more), these HDPE-rich blends may have a constant peak temperature while the deflection point is varied linearly with PS content.