In recent years, several Be- Nb rare- element pegmatite dikes have been discovered in the Ningshan district of the Qinling orogenic belt through geological exploration. This study used scanning electron microscopy and electron probe microanalysis to investigate the mineralogical characteristics and mineral composition of monazite in Be- Nb pegmatite. The results indicate that monazite can be divided into two types. The first type mainly occurs in the rock- forming minerals such as quartz and feldspar. Most of the first type monazite exhibit internally homogeneous backscattered electron (BSE) images, while a small portion may show oscillatory zoning. The first type monazite has a higher ThO2 content (7.0%~13.9%, averaging 10. 5%). In contrast, the second type monazite also exhibits internally homogeneous BSE images, but with noticeably lower brightness compared to the first type. The second type monazite has a lower ThO2 content (0.9%~4.4%, averaging 2.2%). This indicates that the first type of monazite has a magmatic origin, while the second type has a hydrothermal origin. Higher concentrations of ThO2 in the primary magmatic monazite are associated with an increase in the brabantite molecule substitution. The U- Pb dating of the magmatic monazite yielded an age of 200.8±2.1 Ma, representing the age of the pegmatite emplacement, which is close to that of the monzonite granite in the Ningshan batholith. The εNd values of both the magmatic and hydrothermal monazite range from 4.6 to 3.0, which is consistent with the εNd value range of the monzonite granite. This indicates the genetic relationship between the Be- Nb pegmatite and the monzonite granite. Based on various lines of evidence, this study concludes that the formation of Be- Nb rare- element pegmatites in the Ningshan district is the result of the residual magma crystallization of the co- genetic monzonite granite magma differentiation in the early stage in the Ningshan batholith.