Abstract: The Nanping granitic pegmatite is one of the most important rare-metal deposits in China. In this pegmatite field the No.31 granitic pegmatite is the most highly evolved and well mineralized one, where beryl is the most important beryllium mineral. The beryls were studied in this paper by using the electron microprobe analyses and back scattered-electron imaging. The results show that the beryls in this pegmatite can be distinguished into primary beryl and late beryl. The primary beryl crystals were crystallized early from the pegmatitic magma in association with rock forming minerals such as quartz, muscovite, albite and spodumene.The late beryl is typically associated with the primary beryl, and was either crystallized from the Be-rich hydrothermal fluids, or the replacement product of Cs-rich primary beryl by hydrothermal fluids along its cleavages, fissures or porosities. The electronmicroprobe results reveal that the primary beryl is also enriched in Fe, Mg, Na, Cs, and contains up to 0.28% FeO, 0.89% MgO, 1.36% Na2O and 3.92% Cs2O. Overall, in the primary beryl crystals from zone I to zone IV, the contents of FeO and MgO decrease, butcontents of Na2O and Cs2O increase. Especially, the contents of Cs2O in the primary beryl show wide variations. Some primary beryl crystals are found in spodumene in zones III and IV, and are enriched in Fe and Mg. The late secondary beryls with low contents of Cs2O are attributed to crystallization of pullucite and nanpingite. There is possibly a re-equilibrium between the late hydrothermal fluids and the Cs-rich beryl.