The study first focuses on analyzing the features and related functions of mobile learning, and then, moves on to review the learning strategies of English vocabulary to form the theoretical basis for the mobile English vocabulary learning system developed in the study. 103 seventh-graders from 3 intact classes in a junior high school in Taipei City were recruited. The three classes were randomly assigned to three different treatments. Experimental group A learned English vocabulary through the mobile learning system with the function of formative assessment. Experimental group B learned English vocabulary through the same system but without the function of formative assessment. Finally, the control group received traditional classroom-based teaching of the same English vocabulary. The differences of learners' achievements, vocabulary retention, and learning process were compared and analyzed.