The effects of the use of cochlear implant (CI) on speech intelligibility, speaking rate, and vowel formant characteristics and the relationships between speech intelligibility, speaking rate, and vowel formant characteristics for children are clinically important. The purposes of this study were to report on the comparisons for speaking rate and vowel space area, and their relationship with speech intelligibility, between 24 Mandarin-speaking children with CI and 24 age-sex-education level matched normal hearing (NH) controls. Participants were audio recorded as they read a designed Mandarin intelligibility test, repeated prolongation of each of the three point vowels /i/, /a/, and /u/ five times, and repeated each of three sentences carrying one point vowel five times. Compared to the NH group, the CI group exhibited: (1) mild-to-moderate speech intelligibility impairment; (2) significantly reduced speaking rate mainly due to significantly longer inter-word pauses and larger pause proportion; and (3) significantly less vowel reduction in the horizontal dimension in sustained vowel phonation. The limitations of speech intelligibility development in children after cochlear implantation were related to atypical patterns and to a smaller degree in vowel reduction and slower speaking rate resulting from less efficient articulatory movement transition.