This paper aimed to investigate the effect of a mobile app on mathematics learning in authentic contexts. Authentic contexts contain rich resources wherein students can use authentic objects as aid in advanced educational technology-assisted mathematics learning. We designed Ubiquitous-Fraction (U-Fraction), a mobile application that helps students to learn fraction in authentic contexts and that integrates fraction concepts into daily contexts. This mixed study involved 80 fifth-grade Taiwanese students. We compared the learning achievements of students who used U-Fraction (experimental group) with those of students who used web-based tool and the traditional learning mode (control groups). We studied the students' behaviors toward the use of U-Fraction in their learning activities and analyzed the relationship of students' learning behavior with their learning achievements. Furthermore, we collected students' perception of U-Fraction. Results demonstrated that the experimental group significantly outperformed the control groups in terms of learning achievements. In terms of learning behaviors, fraction concept/comparison learning with authentic photo taking and fraction addition/subtraction learning with authentic photo taking were most significantly correlated with learning achievement. It indicated that fraction can be learned better in an authentic environment. Furthermore, regression analysis showed that the assignment scores could significantly predict learning achievements. We found that most participants had positive perception toward U-Fraction and that they enjoyed using the photo-taking feature to solve fraction problems in their surroundings. Therefore, we suggest that fraction learning activities should be designed in a way that students are encouraged to apply through photo taking in authentic environments and the knowledge they have learned.