During August of 2012, the research team carried out a field exploration of the Ulanbattar-Xilinhot Grassland Transect on the Mongolian Plateau. The transect was split into three parts according to the physical geographical conditions and practical administrative boundaries: the northern section (from Ulanbattar to Airag), the central section (from Airag to Sunitezuoqi), and the southern section (from Sunitezuoqi to Xilinhot). Along the 1100 km transect, 46 sample areas, 136 grass yield quadrats, and about 500 species-frequency plots were surveyed in detail and a series of species number data and above ground biomass data were then measured. Using GIS software and the GHCN datasets (The Global Historical Climatology Network), the historical meteorological data along the above transects were then extracted. The monthly average temperatures and total precipitation during the summer season (i.e., June, July and August) were calculated. Based on the vegetation characteristics data and hydro-thermic factor data, the correlation and partial correlation were analysed. The results show that: (1) the distribution of monthly average temperatures in summer are presented as an inverted "U" pattern along the transect, i.e., the northern and southern sections showing lower temperatures, while the central section maintained a high temperature level. Conversely, the distribution of total precipitation in summer is shown as an upstanding "U" pattern, i.e., high precipitation in the northern and southern sections, with low precipitation in the central section; (2) the distribution of numbers of plant species and above ground biomass are both depicted as an upstanding "U" shape along the transect. In the southern and northern regions, where the ecological type is typically warm steppe, both the bio-diversity and aboveground biomass are significantly better than those in the central region, where the ecological type is usually warm temperate desert grassland or temperate desert; (3) simple correlation analysis normally reflects the relationship between vegetation characteristics and hydro-thermic environmental factors over large scales (for example, plateau transect). In this study, the numbers of plant species and above ground biomass are negatively correlated with the summer monthly average temperature and are positively correlated with the summer precipitation; (4) the partial correlation analysis is inclined to reflect the relationship among different factors over a local scale (for example, small watershed). In this study, both the temperature and precipitation factors have positive effects on the number of plant species and the ground biomass.