Background: Constipation is one of the most common complaints in children. Guidelines on functional constipation recommend digital rectal examination (DRE) when constipation is suspected. However, diagnosis of megarectum by ultrasonography would differentiate children with constipation from those with dysfunctional defecation. Objectives: In this research, we evaluated the utilization of ultrasonography to measure the diameter of rectal ampulla for the diagnosis of functional constipation. Methods: In this study, 94 patients < 14 years old diagnosed with functional constipation were included. Patients were examined by both DRE and ultrasonography before and after a conventional stool softener treatment. Results: The diameter of the rectal ampulla was significantly wider in patients with large stool mass in DRE than in patients with normal digital rectal exams. There was a significant relationship between fecal incontinence and pre-treatment DRA. By increasing the severity of fecal incontinence, the average DRA in patients increased significantly. Additionally, there was a significant statistical difference between the patient’s DRA before and after treatment. Finally, the relationship between constipation and DRA adjusted model showed that the risk of abnormal DRA was 3.1 times larger in patients with three and four symptoms than in patients with two symptoms and this relationship was statistically significant. Conclusions: Ultrasonography can be a suitable replacement for DRE; however, further investigations are required.