We report here the preparation of Na0.5MnO2 whiskers from Mn2O3 and Na2CO3 in NaCl flux, where NaCl also acted as a reactant along with Na2CO3. As a result, the reaction system became more homogeneous, and meanwhile Mn2O3 reacted with NaCl more slowly than Na2CO3, finally leading to the crystals of high crystallinity and purity. Moreover, large Na0.5MnO2 whiskers (0.01 x 0.01 x 0.1 mm(3)) can be obtained in absence of Na2CO3 due to the slow reaction rate. A detailed study of Na0.5MnO2 whisker growth process revealed that a layer-to-tunnel structure transformation occurred accompanying anisotropic growth and oriented aggregation. (C) 2011 Elsevier B.V. All rights reserved.