Contactless electroreflectance（CER）spectra of c-plane ZnO bulk have been measured from 20 to 300 K. Photoreflectance（PR）spectrum of ZnO bulk has also been measured at 300 K. The CER spectrum is red-shifted with respect to the PR spectrum at 300 K. It is known that the average field experienced in the CER is larger than that of the PR. In addition, the features of PR or CER is blue-shifted when the field becomes larger if they belong to band-to-band transitions. Hence, the observed features are excluded from band-to-band transitions and are attributed to excitonic transitions. The CER spectra were then fitted by Lorentzian profiles. The energies of the A（B）, B（A）, and C excitonic transitions at 0K, which were obtained by the best fit of Varshni’s equation, are 3.419, 3.436, and 3.482 eV, respectively.