In the present study, the ultrasonic data of two prelingual deaf participants were collected to observe tongue movements during the production of all the apical syllables under four citation tones except for \emph{ri} in Mandarin Chinese. Results of data analysis showed that, besides their personal characteristics, the two participants share similar problems in producing those apical syllables such as producing alveolar syllables as post-alveolar syllables, realizing affricates as fricatives, and unable to pronounce some types of apical syllables which they can perceive correctly. Overall, the results of the present study indicate that ultrasound is a promising technology in studying speech production of deaf mutus.