Niobium pentoxide reacts actively with concentrate NaOH solution under hydrothermal conditionsat as low as 120
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C. The reaction ruptures the corner-sharing of NbO
7 decahedra and NbO
6 octahedra inthe reactant Nb
2O
5, yielding various niobates, and the structure and composition of the niobates dependon the reaction temperature and time. The morphological evolution of the solid products in the reaction at180
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C is monitored via SEM: the fine Nb
2O
5 powder aggregates first to irregular bars, and then niobatefibers with an aspect ratio of hundreds form. The fibers are microporous molecular sieve with a monocliniclattice, Na
2Nb
2O
6·
2/
3H
2O. The fibers are a metastable intermediate of this reaction, and they completelyconvert to the final product NaNbO
3 cubes in the prolonged reaction of 1 h. This study demonstrates thatby carefully optimizing the reaction condition, we can selectively fabricate niobate structures of high purity,including the delicate microporous fibers, through a direct reaction between concentrated NaOH solutionand Nb
2O
5. This synthesis route is simple and suitable for the large-scale production of the fibers. Thereaction first yields poorly crystallized niobates consisting of edge-sharing NbO
6 octahedra, and then themicroporous fibers crystallize and grow by assembling NbO
6 octahedra or clusters of NbO
6 octahedra andNaO
6 units. Thus, the selection of the fibril or cubic product is achieved by control of reaction kinetics.Finally, niobates with different structures exhibit remarkable differences in light absorption and photoluminescence properties. Therefore, this study is of importance for developing new functional materials bythe wet-chemistry process.