We have used a nonrefrigerated cadmium trap in a 2-stage glass mercury diffusion pump vacuum system. Experiments show that the cadmium trap is capable of isolating mercury vapour effectively, as high vacua of 10-5-10-7 mmHg is attainable in an evacuated container. Systematic investigations of the effect of the cadmium film under various conditions on the ultimate pressure were carried out. It is found that newly evaporated cadmium films are most effective for that purpose, while after exposure in air of 10-2 mm Hg, the ultimate pressure does not change appreciably, and that had the cadmium film been exposed to the atmosphere, it would be one order higher than before. Improvement on the ultimate vacuum can be effected by decreasing the cadmium trap temperature and utilizing two or more cadmium traps in series as well as through bake-out of the evacuated container. When such measures are put in force, an ultra-high vacuum of 10-9 mm Hg is obtained in our test.