Room-temperature contactless electroreflectance (CER) was used to investigate the optical properties of a N,N′-didecylperylene-3,4,9,10- tetracarboxylic diimide (PTCDI-C10H21) thin-film sandwiched between indium tin oxide and aluminum electrodes (Al/PTCDI/ITO/glass substrate) under vacuum conditions. The electromodulated optical responses of the Al/PTCDI/ITO/glass structures were characterized by various alternating current biases. The optical transitions of PTCDI were perturbed by energy shifts of electronic states due to the Stark effect induced by the modulated electric field. The modulated CER spectrum of PTCDI is strongly enhanced by performing first derivatives on the absorption spectrum of PTCDI. The CER spectrum involves fundamental transitions, doping states, and Davydov splitting. Moreover, the intensity of the field-induced transition peak of PTCDI increases with increasing CER modulation voltage. The transition energies between the lowest unoccupied molecular orbital and highest occupied molecular orbital of the Al/PTCDI/ITO system is obtained from the peak positions in the CER spectrum.