Particle Image Velocimetry(PIV) is a method for measuring velocity distribution of fluctuating water flow or air flow through the digital image processing of multi-couples of images obtained by flow visualization. Using the PIV, velocities at many points in a series of digital images could be easily measured without inserting a measuring probe into flow, which results in a drastic reduction in measuring hours. The PIV also has secondary adovantage in that an experimenter can visually confirm flow patterns in measuring velocity distribution. Despite these advantages, sit is not easy to obtain the velocity measurement parameters in the PIV system. In this paper, two measurement parameters used in the density correlation method and the gray level difference accumulation method of the PIV system, an interrogation region and a search region, were experimentally investigated by comparing the velocity distributions measured by a Laser Doppler Anemometer(LDA) and the PIV. An open channnel backward-facing step flow was used for the investigation because this flow has a great velocity distributions in a relatively short experimental section. The interrogation region is a square frame of pixels, and a two-dimensional distribution pattern of gray level in this region is compared with that in its neighbors in the sensity correlation method. The search region is a square frame of pixels in which the similar pattern to that in the interrogation region is searched in the density correlation method. The results indicated that the interrogation region should have an adequate size of pixels in which a definite distribution pattern of gray level was secured. The search region should be determined by considering the maximum velocity, the frame rate of CCD and the spatial resolution of images.