This paper put forward a new method of observing tongue movement in ultrasound images by calculating the accelerations of tongue points during speech production. By applying this method, the current study further compared the tongue movements in producing Mandarin mid-vowel [τ], the so-called transitional vowel with those of two steady monophthongs [i] and [a] in Mandarin Chinese. The results revealed that the tongue involves more movements in pronouncing [τ] than [i] and [a], which physiologically supported the notion that [τ] is a transitional vowel in Mandarin Chinese. The accelerations of tongue points could be served as promising indexes of describing tongue movements in speech articulation.