Abstract: The activity, specific rate of reaction and usable ratio of inner surface (URIS) for commercial A301 ammonia catalyst were measured in a tube packed bed reactor. The reaction conditions were: pressure of 7.0 , 10.0 and 15.0MPa, temperature of 325—450℃, space velocity of 6000, 8000 and 10000h ^-1 and particle size of 0.6—0.9,1.6—2.5,2.5—3.0, 3.0—4.0 and 4.0—6.7 mm. The outlet ammonia concentration, specific rate and URIS decreased with increasing particle size. URIS of commercial catalyst depended not only on particle size, but also on temperature and catalyst efficiency. Particle size was the most important factor for URIS. With increasing particle size from 0.6—0.9mm to 4.0—6.7mm, URIS of catalyst decreased from 1 to 0.74—0.80. For the same size range of catalyst,URIS depended on temperature and catalyst efficiency. Above the temperature of 350℃,catalyst efficiency and URIS increased with rising temperature. But below 350℃,URIS increased with the decrease of temperature.