A series of benzoxazole derivatives are revealed to exhibit electrochromic phenomena, and the three primary colors such as red, green and blue are obtained by means of changing the structure of the benzoxazole derivatives. The coloring mechanism of these derivatives is evaluated with electrochemical and spectroscopical measurements. Phase-selective second-harmonic AC voltammetry is applied to obtain the reversible half-wave potential ( E ^γ_1/2) of benzoxazole derivatives. This measurement enables to evaluate E ^γ_1/2 more correctly than ordinary cyclic voltammetry under the system including the follow-up chemical reaction. From the results obtained with these methods, the coloration is probably caused by the interaction between the benzoxazole derivatives and supporting electrolyte and/or solvent. Benzoxazole derivatives containing thiophene ring exhibit clectrochromic behavior with low eletrochemical energy than that containing phenyl ring. This is attributed to a difference of resonance energy between thiophene ring and phenyl ring. Relation between electrochemical and spectroscopical data is also discussed.