Previous studies on electronic textbooks were all survey studies of college students in the U.S.A. Their main findings revealed that using electronic textbooks and using print textbooks showed no statistical differences in learning achievements. As mobile devices became popular in classrooms, the present study was intended to confirm the competitive nature of electronic textbooks on mobile devices in an experiment. The differences of the present experiment from previous survey studies included its involving teenage learners of English and its examining their learning with electronic textbooks on mobile devices. This experimental study recruited adolescent learners to participate in a mobile-assisted English learning program requiring them to study with their electronic textbooks on mobile devices exclusively. Their counterparts used print textbooks as usual. The findings presented no significant differences in various achievements between the two groups. The mobile group furthermore approved their studying English on their mobile devices with satisfaction and perceived its usefulness and ease of use. Endorsing the competitiveness of electronic textbooks on mobile devices, the teenage English learners also highlighted the strengths characterized by mobile technologies.