Based on the dynamic frame of mesoscale model MM5 and Reisner2 explicit cloud scheme, a LN (liquid nitrogen) -seeding scheme was developed and used to simulate the cold fog dissipation for the cold fog event on 26 December 2007 in Beijing area. The seeding effect and the physical mechanism were studied. Furthermore, two sensitive experiments were performed to study the seeding effect under different seeding distances and seeding amounts. The results indicate that when the seeding operation lasts 10 min at seeding rate of 5g/s with distance being about 1－2 km on the upwind side of the target area, the seeding effect in the target area begins to appear at 9 min after seeding operation and the best effect appears at 24 min after seeding operation, the seeding effect can last about 20 min. The microphysical mechanism of the cold fog dissipation is because of the depletion of the water vapor due to the ice sublimation growth induced by seeding, which leads to the decrease of the water vapor condensation into fog droplets, meanwhile the decrease of the fog advection from the upstream region after seeding operation also contributes to the fog dissipation. In this case, seeding operation at seeding rate of 15 g/s with distance being about 5－6 km on the upwind side of the target area can make the target area have the most significant improvement in the visibility.