In the present study, the phase/Doppler particle sizing technique have been carefully reviewed in order to understand its limits of application and measurement accuracy. The phase/Doppler technique is based on the measurement of the fringe pattern formed by spray drops passing through the measuring volume at the intersection of two laser beams. The phase difference between the scattered light signals detected at two different positions is proportional to the drop size, and also the signal frequency is proportional to the drop velocity. In using the phase/Doppler technique, the signal quality and measuring accuracy can be improved with the larger collecting lens and the forward scattering mode, and of course, with the accurate optical alignment. Another important factor, other than the optical setup, is the signal processing technique. The cross-spectral density function was used to evaluate the phase difference and the Doppler frequency, with which the size and velocity of a drop(particle) can be obtained even at the low SNR(Signal to Noise Ratio) condition. Experimental results show that the particle size can be measured with a reasonable accuracy when the forward scattering mode was chosen. However, when measuring transparent particles with the backscattering mode, the phase/size relationship cannot be expressed in a closed form mathematically, and the numerical calculation should be performed to obtain the relationship; the closed form relationship for the backscattering mode is possible only with the opaque(non-transparent) particles. To measure the number density of particles accurately, the effective measuring volume should be determined appropriately, which takes account of the Gaussian distribution of the laser light intensity.