The results of field experiment in 17—18 Dec.1997 and indoor little cloud chambers experiment conducted by Weather Modification Institute of Chinese Academy of Meteorological Sciences and Peking University in 1994—1995 are analyzed. It is shown that whether indoor or in the field, liquid nitrogen is an ideal catalyst because it obviously affects the visibility well. The microphysical structure observed at a site shows when successively seeding liquid nitrogen into the natural super-cooled fog whose horizontal visibility is 50—100m and number concentration is 1206/cm³, ice crystal will appear at the leeward side of the observation station about 4.5—5km after 35 minutes, it is mainly columnar, and its mean concentration is 0.35/cm³. The appearances of ice crystal disputed the stability of fog colloid structure, and changed the visibility well. When liquid nitrogen was seeded the second circle, wind direction turns to 210°—240°(NNW), no ice crystal appeared, concentration of fog drop decreased a little, small one decreased, and spectrum turned wide.