Trace elements of potassium feldspar (Kfs) from pegmatite veins in the Keketuohai pegmatite district were determined in situ by LA-ICP-MS. As the K/Rb ratio decreases, the contents of compatible Ba and Sr decrease, while the contents of incompatible Li, Cs and Ga increase. The K/Rb ratios and above mentioned trace element contents of Kfs from the Be-Nb-Ta mineralized pegmatite veins match those of Kfs from barren pegmatite veins. In contrast, the mineralized pegmatites are characterized by having K-feldspars with high phosphorus contents, mostly above 0.1 wt% P2O5, which is the boundary separating mineralized pegmatites and barren pegmatites. The high P2O5 content in Kfs indicates high concentration of this element in the melt from which these pegmatites crystallized. The P2O5 content in K-feldspar can be used as an exploration tool for LCT type Be-Nb-Ta deposit.