We describe the development, implementation and evaluation of an augmented reality (AR) application for mobile devices. It has been particularly intended to be used in educational environments. This application has been called U-AR, and is based on optical image recognition from real environment images, to act as markers. Once they are associated with virtual information (3D architectural models, in our case), the application allows viewing, scaling, and positioning it interactively and in real time. Some improvements have been implemented compared to existing commercial ones, such as the ability to view different models consecutively with a single marker and the possibility to move these computer-generated objects around the scene. The main objective was to assess the feasibility of using AR on mobile devices in educational environments. In addition we evaluated academic performance improvement using this technology. Validation was done through a case of study. It was carried out by Architecture students of the ETSAB, (UPC-Barcelona-Tech) outdoor. Results obtained by student's architectural proposals, after using this technology, and by questionnaires responses, demonstrated the application suitability as a new tool to be used in learning processes.