This paper presents an acoustic study which contributes to depicting the acoustic cues of Mandarin Chinese tones in whispered speech. By analysing modifications of the acoustic properties of the vowel /a/, we have found a correlation between the duration of vocalic segments and tones, in both normal and whispered voice. The durational differences among vowels which carry different tones are more noticable in whispered voice than in normal voice. In addition, while observing intensity and formants trajectories, we have uncovered a curve pattern which highlights the relationship between tones, intensity and formants trajectories in whispered speech. This pattern suggests that vowel intensity and formants may carry tonal information in a coordinated manner.