Ludendorff's catalog of the M13 globular cluster results from observations made with the 32.5cm refractor of Potsdam Observatory, one on 17 July 1900 (plate A), and the second on June 1902 (plate B). Each plate was measured twice, in opposite position, to remove the possible errors. The catalog contains accurate positions on both plates, photographic magnitudes, and comparison with the previous results from Scheiner (1892). Number 1119 to 1127 where seen only in one position on plate A, and in two positions on plate B. Number 1128 to 1136 where seen only on plate A, and are not in Scheiner catalog.