Comprehensive field investigations had been conducted for three times on Nam Co, central Tibet during 2005-2007. Here we presented the preliminary results focusing on bathymetric survey and water quality measurements. Isobathic map showed that Nam Co is a high altitude deep lake where a flat and large basin lies in the central part with more than 90m water depth. Data on water depth in northwestern bank areas of Nam Co provided unquestionable evidence for water level rising in the recent three decades due to the formation of two small islands which were peninsulas in 1970s. Water quality at 16 stations covering more than half of the whole lake showed that temperature, pH, dissolved oxygen, conductivity and photosynthetically active radiation (PAR) of surface water was averagely 11.63℃, 9.13, 7.93mg/L, 1839μS/cm and 2582μmol/(s·m²), respectively. According to the vertical variations of water quality parameters, water column in relatively deep lake areas of Nam Co could be divided into three layers with distinctly various features. The first layer was from surface to about 18~20m depth in which parameters was stable with higher temperature and abundant sunlight;the second layer ranged about 20m to 60m where thermocline developed;and the bottommost layer was characterized as also very stable with low temperature and no light, forming a cold and dark underwater world.