The phosphorus contents of alkali feldspars of 80 pegmatite veins from the Chinese Altay, NW China, were determined by electron microprobe in this study. The alkali feldspars of the barren pegmatites are poor in phosphorus and range from <0.01–0.05 wt% P 2 O 5 . Alkali feldspars from the fertile (rare-metal mineralized) LCT-class pegmatites are characterized by P 2 O 5 in the range 0.10–0.40 wt% P 2 O 5 for K-feldspars (Kfs), and 0.10–0.30 wt% P 2 O 5 for albites (Ab). In the Qunkuer Be-Nb-Ta mineralized pegmatite, a value of 2.89 wt% P 2 O 5 in K-feldspars was determined, the highest value of P 2 O 5 in alkali feldspars reported. According to the statistical results, 0.1 wt% P 2 O 5 in alkali feldspars separate the fertile pegmatites from the barren pegmatites, and more than 0.1 wt% P 2 O 5 in the alkali feldspars, especially in K-feldspars, can be used as an indicator for prospecting for LCT-class pegmatite-type rare-metal ore deposits.