The photon correlation spectroscopy (PCS) spectrum obtained from an actual dynamic light scattering experiment for particle sizing is usually a mixture of self-beating mode and heterodyne mode. Simulation indicates that a large error can occur when the size of the scattering particles is inverted from such a mixed PCS if pure self-beating mode or pure heterodyne mode arithmetic is adopted. A method of inverting the particle diameter from such a mixed PCS signal is introduced. The validity of this method is demonstrated by simulation and PCS experiments with standard particles.