Investigation of geochemistry of surface water is a fundamental study of the regional environment in ungauged areas. Also the study is helpful to determine the relationship between hydrological processes and environmental factors including the climate change and geologic structure. Sampling of the lake and river waters in the northern-central parts of Mongolia was carried out in 2013 and 2014. Then, using the measured chemical data of water solutes, the ion compositions and its controlling factors were analyzed preliminarily. The traditional methods in geochemistry, such as Gibbs correlation analysis and Stoichiometry, were applied in this study. Results indicate that the waters in the study area are from slightly alkaline to alkaline. The average value of total dissolved solids in river waters is about 114.0 mg/L, which presents a significantly different spatial variation of ion compositions in river waters. However, generally Ca2 + and HCO_3~- are the dominant ions in the surface waters in the study area. The most important source of the solutes in the surface waters is rock weathering process. In particular, the contribution of carbonate and sulfate minerals weathering are the largest. Although the current water quality is in a good situation in the northern- central parts of Mongolia, an accumulating trend of the nitrate in waters has been detected. Therefore, great attention should be paid to the influence from the anthropogenic input.