Basic magnesium chloride (BMC) whiskers were prepared by hydrothermal method using magnesium chloride and calcium hydroxide as raw materials. Subsequently, nano-MgO was obtained by pyrolysis of the resulting whisker. Transmission electron microscopy (TEM) images showed that the diameter of nano-MgO was between 20 and 40 nm. The corresponding selected-area electron diffraction (SAED) pattern demonstrated the single-crystalline nature with the exposed plane of {111} and {110}. TG-DTG, SEM, XRD, and FT-IR analysis confirmed the four-step pyrolysis processes of BMC whisker. Results showed two molecules of crystal water lost during the first and second steps, respectively. Subsequently, two hydrogen chloride molecules were released in the third step. Finally, hydroxy water was dehydrated. The thermal decomposition kinetics of BMC whisker was studied by means of Satava and differential method. It indicates that the first step of the thermal decomposition of BMC whiskers is random nucleation and subsequent growth mechanism. The second step is two-dimensional diffusion. The third and the final steps are phase boundary reaction and one-dimensional phase boundary reaction mechanism, respectively.