This paper introduces the laser speckle in laser display system and autocorrelation function to measure the speckle size. What's more, it develops a new method to calculate the speckle size. It firstly thresholds the speckle image into binarization image. Then it calculates the statistical average of run-length of the speckle in a given direction. This statistical average of run- length can describe the speckle size precisely. Finally, it draws a conclusion that the new method is much easier for hardware implementation comparing with that by autocorrelation function.