Desert, desert-steppe (Gobi) zone is the largest province by area (165,000 square kilometres) of Mongolia. The Gobi contains some 560 plant species, in 80 different vegetation associations (Swift, 1991). There are number of results of nutritive value of desert-steppe’s pasture grass. Mongolia’s dry climate means that pasture grasses have less moisture, and are richer in protein, carbohydrate, vitamin and minerals. Such an ecosystem is more typical of the Gobi zone than Khangai mountain zone (Tserendulam, 1980). However, have not been investigated seasonal changes of CP, NDF, ADF, Hemicellulose, and in vitro and in sacco degradability all together of desert-steppe’s pasture grasses throughout the year. The objectives of this study were to determine the chemical content and the rumen degradability (in sacco & in vitro) of desert-steppe’s 3 common pastures. Our investigation demonstrated that, chemical content and in vitro gas production of desert-steppe’s pasture grass were variable across a year depending on stage of the maturity of the plants. As example, CP content of pasture grass ranged 4.28-15.13%, NDF 55.05-71.96%, ADF 33.96-48.45%, hemicellulose 17.11-29.79%. Cumulative gas production after 70 h in vitro fermentation was accounted 143.10 ml/g to 266.19 ml/g. DMD was ranged 54.73- 77.71%, OMD was ranged 50.18- 77.97%. The potential (a+b) of in sacco degradation of pasture grass ranged from 58.7-76.8%. The rate constant (c) of degradation of dry matter was very changeable (0.0269-0.0934 %/h) throughout the year.