Sustained overgrazing and rodent’s destruction are responsible for the degradation of more than 50% of grassland in north-western China. In fact these persistent adverse effects caused a reduction in the number of high quality forage species and grassland productivity, accompanied with an increase in the incidence of weed and poisonous plants. The degradation of the grassland ecosystem causes serious social and economical problems in the region. To prevent and control grassland degradation, intensive multidisciplinary research projects have been performed in this area. However information on the microbial population changes in the grassland is scarce. Soil microbial activity plays an important and vital role in grassland ecosystem because it affect soil physical properties like the stability of aggregates, soil aeration and water holding capacity, and it is responsible of the mineralization and recycling of several important nutrients like carbon, nitrogen and phosphorus. Soil organic matter is also significantly affected by microbial activity. Thus, grassland soil microbial importance and diversity can be used as an index of grassland health and soil fertility. The present work was performed in the alpine region of Tianzhu, eastern part of the Qinghai-Tibetan Plateau, which is characterized by high altitude, important daily temperature variations with sparse air, strong radiation and low temperature. We studied the temporal changes in vegetation and soil properties, the different physiological groups of soil microorganisms and the effect of the degree of grassland deterioration on soil microbes, by comparing measurements made in 1982 with those made in 2003. In general, the vegetation condition as well as soil physical and chemical characteristics were obviously in better condition in 1982 as compared to 2003. In 2003, the parameters showing important decreases as compared to 1982, included: the dominant plant species of the vegetation coverage, forage yield, soil water content, soil organic matter, N and P content. On the contrary an increase in soil pH was observed. The total microbial population in 1982 was higher than in 2003. Soil bacterial population decreased from log 8.97 cfu g-1 of dry soil in 1982 to log 6.79 in 2003. Similarly from 1982 to 2003, the observed population decreases in cfu g-1 of dry soil were as follows: actinomycetes from log 6.91 to log 6.17; fungi from log 3.89 to log 3.28; nitrifiers from log 4.29 to log 3.54; aerobic nitrogen fixing bacteria from log 5.54 to log 3.90 and cellulose degraders from log 5.65 to log 3.68.The number of physiological groups was also from 1.5 to 4.5 times higher in the moderately deteriorated grassland (fenced) than in the severely deteriorated grassland (unfenced and rodents hill). The results clearly illustrate the important decline of the soil microbial population associated with the deterioration of the grassland.