The zero offset experiments for pyranometers were done outside in the night of clear day using the comparison method. On the basis of the experiments, the zero offsets of different pyranometers (the blackwhite pyranometers and allblack pyranometers, the homemade and imported pyranometers) were compared respectively. The causes for zero offset were analyzed. By analysis of the experimental data, it was shown that the zero offset of the homemade pyranometers was less than 10 W·m-2 in the night of clear day. The experiments, in which the sensing surface of pyrnaometers was installed downwards and a lid was put on the pyranometers, all demonstrated that the zero offset was caused by the cold sky effect indeed.