In particle Image velocimetry, a thin laser beam is usually used to interrogate the double-exposure photographic image point by point in order to find out displacement of the particles. In this paper, the relation between the choice of interrogation region size and the displacement of the measured particles is daduced by analyzing the effect of Size of the region upon the speckle distribution and the influence of the speckle noise upon the disc-rimination of the Young's fringes. The problem that the size of the interrogation region places a restriction on the maximum displacement to be able to measure and the criteria for reasonablly choosing the interrogation region are discussed and analyzed.