It is frequently reported that early repair of the soft palate induces narrowing of the remaining palatal cleft and thus facilitates later hard palate closure. However, to the best of our knowledge, there have been no comparative studies to test this hypothesis. The purpose of this retrospective study was to evaluate the change of palatoalveolar morphology following primary lip repair and posterior palatoplasty. Dental plaster models of patients with complete unilateral cleft of lip and palate (UCLP) were used to measure the width of the cleft and palatal arch. Twenty-six patients received simple posterior palatoplasty (PP group) simultaneous with primary lip repair, and 20 patients did not (NPP group). The dental models included one preoperative cast at 2 months (T1) and two or three casts at 6 (T2), 12 (T3), and 18 (T4) months before final palate closure. The linear measurements performed were width of alveolar cleft (Ca); width of palatal cleft between the canines (Cc), molars (Cm), and tuberosities (Ct); the palatal arch distance between the canines (Dc); the widest distance between molars (Dm) and the tuberosities (Dt); and the palatal height between the canines (Hc) and tuberosities (Ht). The raw measurements and the calculated cleft-to-arch ratios of Cc/Dc, Cm/Dm, and Ct/Dt were compared between the two groups. The results showed gradual narrowing of the width of cleft from T1 to T4. Narrowing of alveolar cleft width (Ca) from T1 to T2 was dramatic. The palatal arch (Dc, Dm, Dt) showed no change to mild increase in width. The cleft-to-arch ratios decreased with time. The palatal height remained the same or slightly increased over time. There were no significant differences observed between the PP and NPP groups among these measurements except for the Ct and Ct/Dt at T4. In conclusion, after initial lip repair, there was a decrease of the width of cleft in patients with complete UCLP during the 18-month period, and simple posterior palatoplasty did not further narrow the cleft nor influence palatal arch development.