A differential approach is proposed for the determination of initial onset in differential scanning calorimeter (DSC) diagram of phase change material (PCM). By dissecting the spacing between extrapolated onset determined with traditional method and origin attributes to various delay mechanisms. A position-dependent term added into the traditional energy change equation accounts for thermal delay from any moving interfaces between separated phases of PCM inside a DSC cell pan. With the aid of Euler's equation in calculus of variations, the heat flow rate equation in relation to three major delay functions and temperature is formulated using chain rule of variables. The second-order differential operation with regard to heat flow rate in DSC diagram helps shortening the distance from extrapolated onset to origin, so as to initial onset can accurately located. The rampant fluctuation in the second-order derivative of heat flow rate in DSC diagram vindicates inadequate compensation of control unit in DSC, a result of serious feedback delay due to poor thermal conductivity and large latent heat in PCM. (C) 2008 Elsevier B.V. All rights reserved.