The morphology and phase transformation of CoAl layered double hydroxide (denoted CAL) nanoplates are systematically investigated in highly concentrated basic solution. Characterization results reveal that this is a time-dependent process, in which CALs are gradually converted to β-Co(OH)2 and CoOOH, and the structure becomes increasingly porous. This process could be caused by the dissolution of the Al part of CoAl LDHs, recrystallization of the Co part, and subsequent oxidation of Co. It is found that the pseudocapacitive performance of the products vary with the immersion time. CALs already show a reasonable performance, whereas short-term treatment would lead to the enhanced capacitance of 1123 F g−1 with both remarkable rate capability and stability. However, the formation of CoOOH will significantly degrade the supercapacitive performance.