Introduction: Unilateral functional posterior crossbite (UFCB) is an orofacial discrepancy, which is defined by upper and lower jaw size discrepancy and facial asymmetry. In order to achieve maximal intercuspidation and stable central occlusion the patient’s lower jaw shifts sideways. Irregular orofacial functions lead to altered equilibrium of oral dynamic forces, as low tongue posture and strong buccal muscles inhibit the growth of maxilla. Aim: The purpose of this research work was to create an orthodontic appliance according to Marco Rosa, to present its manufacturing process and its features, to evaluate treatment efficacy and 3D jaw morphology before and after the maxillary expansion using 3D jaw diagnostics. Methods: Foreign and home literature from orthodontic area was reviewed. Practical part of orthodontic appliance manufacturing was performed in specialized orthodontic dental laboratory. 3D morphological features of patient’s jaws were evaluated using program RapidForm. Statistical analysis of planned orthodontic appliance features was made using parametric and non parametic tests. Results: Unilateral functional crossbite treatment with rapid maxillary expansion device according to M. Rosa was successful. The crossbite and other UFCB symptoms were no longer present at the end of the treatment. Morphology of the jaws have significantly changed, jaws sizes at the end of treatment are comparable to average population jaw size. Discussion: UFCB is one of most prevalent and difficult to treat orofacial discrepancies, treatment is therefore planned preferentially. Many different orthodontic appliances are described in literature, which are proposed for treating the UFCB. Results of our study show: deciduous teeth are able to withstand rapid maxillary expansion forces and are therefore suitable for treatment. Modern 3D diagnostics allow us to objectively evaluate jaw morphology. Main advantages of 3D diagnostics are reliability, non-invasiveness and child - friendliness.