An impurity interaction code for a mesh-packed cold trap was developed, and the properties of the packed trap, such as the trapping efficiency and capacity, were calculated using this code. As a result, the trapping efficiency and capacity are greatly affected by the packing ratio and the wire diameter of the mesh packing. Precipitation of impurity is promoted at the entrance to the mesh packing. This facilitating effect hastens flow blockage and decreases the capacity. Moreover, the capacity decreases with increase in the efficiency.